Here you can find some useful technical information and guides pertaining to digital imaging in general and to particular devices and technologies. Enjoy!

Available guides:

• Calibrating your monitor
• Optimizing images for printing

CALIBRATING YOUR MONITOR
THE DEFINITIVE GUIDE

INTRODUCTION

In the world of digital imaging, few things are as important as proper monitor calibration. After all, if what you see on your screen does not match the actual content you are about to send to a printer, all your efforts and expectations are moot.

Worse yet, since we are way into the 21st century now and colour management has been an integral part of professional imaging for close to a decade, it has become increasingly difficult to deal with imaging services providers without having a correct colour setup.
Five years ago service people would make an effort and try to troubleshoot your files, these days they just tell you to take your stuff and bring it back fixed, with profiles embedded.

This guide is intended to help you calibrate your monitor for use with imaging applications, such as Adobe® Photoshop®.
As it is impossible to achieve this task by relying on your eyes only, you will have to use a hardware calibration instrument.

KEYWORDS:
white point, black point, gamma, luminance, brightness, profile, colour temperature, tonal response curve, look-up table (LUT).

Establishing concepts and defining your goals:
Firstly, and most importantly - the modern colour management paradigm assumes a digital image with an embedded colour profile to be a self-sufficient source of colour information.
It does not need to be accompanied by a colour proof, nor be supplied with colour patches or paint chips, or Pantone® reference numbers. It is self-contained, self-describing and complete.

Therefore, your task is to set-up your display in such a way that it can present the information from your image files in the most accurate manner possible. This guide will take you through the steps necessary to achieve this goal.

NOTE: Digital imaging technology changes so fast that it makes little sense to refer to a particular software and/or hardware product. The steps described in this guide are applicable to any modern calibration package on the market.

REMINDERS:

• Allow your monitor to warm up (30-60 min) prior to calibration.
• Make sure there is no side lighting that might be affecting measurements of your hardware calibrator.
• Clean the screen of dust and smudges prior to starting.
  
• Always attach your measurement device to a part of the screen where images are likely to appear, and not a part of the screen that usually displays Photoshop® palettes, etc.
• Remember that you are calibrating not just a monitor but a video-system - that is, a monitor attached to a video-card or other device that generates a video signal - meaning that calibrating a monitor on one computer and then moving it to another computer is not a valid option.

GOOD STARTING SETTINGS:
brightness/Luminance: 120-130 cd/m2
gamma: 2.2
white point temperature: 5500K for daylight balance / lightbox matching

PRACTICAL STEPS

NOTE: This first round of adjustments is performed on a monitor itself, via a set of built-in controls, such as brightness, contrast, etc, available through OSD (On-Screen Display). Most likely, this means pushing a lot of buttons on the front panel of your monitor, though some models come with software that allows you to change monitor settings using computer controls (ie. NEC's NaviSet and Samsung's MagicTune)

1. BRIGHTNESS/LUMINANCE

Unlike CRT monitors of the past, modern LCD monitors are often shipped with extremely bright default settings. This might be a necessary evil when your workstation is located in a bright-lit office or next to a sunny window, but because the majority of imaging systems are set up in subdued, controlled-lighting environments, often enough a monitor brightness needs to lowered to a more reasonable level.

Brightness, or luminance, is measured in candelas/square meter, or cd/m2. It is controlled by the intensity of backlighting located at the back of your LCD monitor, behind the panel itself. The relationship between brightness and contrast in LCD monitors is quite a bit different than in traditional CRT screens. In particular, while LCD contrast is affected by the brightness setting, the opposite is not true - therefore, adjust brightness first, followed by contrast.

Here are some simple guidelines for selecting a correct luminance level for your situation:

• Numerical approach
  Recommended luminance values for professional systems are 90 cd/m2 for CRT monitors (this is a bit ironic, since most CRT monitors cannot manage more than 100 cd/m2 and maintain image integrity anyway); and 120 cd/m2 for LCD systems.
• Comfort approach
  If you work with your computer a lot, you probably have your own sweet spot for a screen brightness, a setting that feels comfortable and does not tire out your eyes easily. From experience, comfortable settings for screen luminance fall in the range of 100 to 140 cd/m2 for controlled lighting environments. Compare this to the fact that many monitors ship with brightness set-up in 250-300 cd/m2 range... - so if you have a new monitor and looking at it feels like looking at the sun, there's nothing wrong with your eyes, it's just too bright!
• Matching to a familiar light-source approach
  If you have a viewing lightbox that you trust, you can use its brightness to set up your computer monitor too. Put a sheet of white paper on a lightbox - preferably a sheet of archival paper without optical brighteners (these papers are betrayed by a blue tint) and adjust your screen brightness to match. Simple and effective.

*An important consideration regarding brightness/luminance:
Many screen manufacturers claim astonishingly high contrast and very vivid colours available in their models. Almost without fail these numbers are measured when monitors are set up to brightness levels which are simply impossible to work with (well, unless you are in a brightly lit office maybe, or on the beach...). Once you have brought the brightness levels down to reasonable values, contrast and the range of available colours take a serious hit, sometimes to the point that a monitor is unsuitable for critical colour work. Generally, high-end professional LCD models maintain better colour palettes and contrast at the luminance levels which are good to work with, but many mid-range models fail at this simple test. It goes without saying that this feature needs to be thoroughly researched before purchasing a new monitor.

Lastly, it is worth mentioning that once you have set your initial luminance level and proceeded to tweak the monitor's white point (more about that in the following paragraph), the luminance/brightness might change enough to warrant an another tweak. So, your typical workflow should go like this:
adjust brightness - adjust white point - adjust brightness again.

2. WHITE POINT

This is a very self explanatory step - what you want is to make sure that the whitest white on your screen looks just right - not too yellow and not too blue and, ideally, matching a standard D50 lightbox or viewing booth if you have one. White point is measured in temperature K (Kelvin). A colour temperature of 5000K refers to the colour of carbon heated up to 5000 Kelvins (that's 4727 degrees centigrade).

Because human vision is very adaptable, a relatively wide range of white point temperatures can be used, all the way from about 4500 to the 9000s. This helps with better integration of the monitor into a lighting situation in a studio. For example, tungsten lighting might require a reddish/yellow white point, and bright fluorescent lighting might need very blue settings of 7500K and up.
The most practical approach is to set-up colour-critical systems in environments with dimmed, neutral lighting and gray walls, so there's a minimum amount of colour pollution.

Selected white point temperature affects the grayscale balance all the way down to the darkest tones, and not just whites or highlights.

Another point worth mentioning is the colour of a desktops wallpaper. Default settings on many popular operating systems feature bright blue desktops. It is recommended to change the desktop colour to a medium gray, since even a small patch of a bright, saturated colour in a corner of your screen might be enough to throw your colour vision off balance.

3. CONTRAST

Having finished with luminance and white point it's time to address the issue of contrast. On LCD monitors changing the contrast settings does not affect the amount of backlighting at all, as contrast is completely decoupled from brightness/luminance. Contrast adjustment on an LCD monitor is just a relic and its primary function is to optimize your video signal when you are connecting your monitor via analog (VGA) connection. Some monitors, in particular the ones from Apple are designed to be used only with a digital connection so they don't have any contrast adjustment at all.

That is because all the contrast-related settings - such as gamma, tonal response curves and, to some degree, grayscale balance, - are better handled by adjustments to a video-card LUT (look-up table), which we'll talk about shortly. Meanwhile, if your monitor has contrast adjustment controls, all you have to do is to find a setting which does not limit or distort the visible tonal range of your monitor in any way.
Some fancier monitors also have variations on contrast control, i.e. many NEC ones have "black level" adjustment. Again, all you need to do is to make sure that it's set to a value that does not compress or limit the range of tones that you monitor can display. Usually this means leaving these at their factory settings.

NOTE: For the most part, the steps described above are performed manually, via on-screen controls( or display's own custom software, such as NaviSet or MagicTune). However, more sophisticated display systems often come with their own calibrator and a set of calibration software that can tune brightness, white point, etc., to whatever settings you want, AUTOMATICALLY. Control signals (i.e "lower brightness 1 step") are sent down to the monitor through the usual DVI or VGA cable, via interface called DDC/CI (dedicated display channel/control interface). The resulting changes on screen are being tracked by a measuring device until the desired values are reached. Here is the example of DDC/CI compliant software from NEC®.

4. CALIBRATION
Up to this point, most of the adjustments are performed on the monitor itself, via its set of On-Screen Display (OSD) controls (or via DDC/CI interface and accompanying software). Now it's time to stop thinking about display only, and start thinking about calibrating your video system as a combination of a video-card with a screen attached to it.

All modern video-card can vary their outputs brightness, contrast, colour balance and the like in a reasonably wide range and in very minute steps. The related settings are stored in the video-cards's built-in "look-up table", or LUT.
Of course, you need to remember that the maximum range of possible colours is determined by the monitor itself, along with the settings which we covered in preceding paragraphs. From now on we'll be making adjustments to the video-cards output only - the process known as calibration. In any modern calibration software this step is performed automatically.
In short, what needs to be done is to measure the displays white point and compare it to the desired value... if there's a difference, one or two colours, ie. Red and Blue might need to be brought down a little until a good match is achieved. Notice that the video-card cannot "brighten" its output, it can only limit it down. For this reason it's always better to try to achieve as precise adjustment to the white point as possible on the monitor itself!

Likewise, if the monitor's blackest black has a blue tint, calibration will have to bring up Red and Green levels until they are even with Blue.
Once the white and black points have been measured and adjusted, calibration software will attempt to adjust the LUT in such a manner that all the in-between shades of gray are lining up smoothly and uniformly, according to preset gamma and/or tonal response curve. Ultimately the gray balance is always determined by the white point temperature, as black always appears neutral. Therefore, for example, choosing a bluish white point will affect all the gray tones down to the darkest ones, and not just the highlights.

5. PROFILING
With manual adjustments and calibration complete, the only thing that's left is profiling. This is achieved by displaying a series of colour patches covering as many different shades of red, blue and green as technically feasible within a limited amount of time; and measuring them with your hardware device. In any modern calibration software this step is performed automatically and is often combined with calibration.

Resulting value pairs (displayed RGB value and actual measured colour) are compiled into what is known as a "device profile".
It is then marked as a default display profile and from here on all the colour displayed by Adobe® Photoshop® will be passing through a colour transformation defined in that profile, so the screen output appears correct.

If you were reading the previous 2 paragraphs carefully you remember that we could only profile our screen after it was calibrated. And since the calibration takes place in a video-card, our profile is valid only when a correct LUT values are loaded into a video-card. On Mac OS X the LUT is saved along with the monitor profile and is loaded automatically each time the system reboots or you change your display colour settings. On Windows PC there is a special program that will upload an LUT after every reboot - it is usually called a "calibration loader" and is installed alongside the main calibration software package.

Note that neither calibration nor profiling will make your display perform better than it physically can - ie. display blacker blacks or more vivid, saturated colours. For example, many mainstream consumer displays cannot show even moderately saturated colours available in sRGB colour space.
However calibration and profiling will ensure a correct display of tonal values, grayscale balance and relationship between colours.

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OPTIMIZING IMAGES FOR PRINTING

The main challenge of mainstream imaging technology, at least when it comes to a colour reproduction, is the fact that the usable dynamic range in photographic capture/output devices is much smaller than the range of colours we can see with our own eyes.

Starting with the camera capture, where some sacrifices need to be made - clipped highlights and shadows, compressed mid-tones; continuing through with specific challenges of digital processing; and ending with limited colour gamut of output devices - each step of the way a little something is lost or thrown out.

The only sensible way to deal with this problem is to take a pro-active approach to optimizing your print images and wring out every little bit of contrast and colour left to us. Experience shows that leaving everything on "full auto" results in prints that are not exciting, at best.

This guide is going to focus on aspects of digital post-processing using Adobe® Photoshop®. The number of ways one can work with colour in Photoshop is quite staggering and is outside of the scope of this guide... but you should have no problem applying these concepts even if you are a beginner.

USING A HISTOGRAM TOOL

This tool is popular for a reason - it is a great way to quickly check the tonal range and balance both in-camera and before printing.
Here are some simple DOs and DON'T's for histogram use.

• DO use histogram to assess overall exposure and density
• DO use histogram to assess white and black points, but DON'T use it (or rather, global Levels command) to adjust them
• DON'T use histogram chart to determine a correct contrast
• DON'T automatically assume that "choppy" histogram means that image is going to print poorly

Let's go through some examples!
Here's a histogram of a balanced image with well-defined shadows and highlights:

And here's a typical underexposed image from a digicam on full-auto. What's going on here?
As you can see, there's a sharp spike on the far right - there's obviously some kind of a point light source in the image, or a catchlight - and it made the camera underexpose the image severely.

Here's the same image with the correct exposure. It's still fairly dark - as you can tell by a strong presense of darker pixels on the left side of the chart - but the rest of the tones are distributed evenly across the range. The catch-lights are gone, but they were nearly gone before too, and it's a small price to pay for better exposure.

Why are we looking at histograms instead of images, you might ask? But this is the point! You are much more likely to make a correct exposure decision while looking at a histogram, rather than a tiny, overbright LCD screen at the back of your camera. I have yet to see an on-camera screen which did not make everything look great!

One thing to keep in mind is that printers in general have much harder time reproducing shadow detail than displays, especially tiny un-calibrated screens that are found on most digicam models. Using a histogram to judge the exposure is an important safety check during both shooting and printing .

OPTIMIZING WHITE AND BLACK POINTS

As mentioned before, the range of tones that can be captured on film or digital media is significantly less than what can be seen with the human eye.
The bad news doesn't stop there, though. A vast majority of printers in use today cannot reproduce these captured colours, or dynamic range - not by a long shot. On a lighter end, the contrast range is determined by the whiteness of the media we are printing on, and on a darker end by the blackness of the inks or dyes.

Thus a simple rule - in order to look the best it can, your image needs to utilize as much of a tonal range - made available to us by technology - as possible.
Yes, there are high-key images that do not contain any dark tones, and a hypothetical shot of a gray lighthouse shrouded in a mist might not have neither highlights nor shadows - but these should be YOUR decisions, and not your camera's.

95% of images, however, do have discernible blacks and whites, and so it is critical to have them set to correct values. What are these values?

Every printing process has its own set of values that are known to be the maximum (or minimum) that still can hold detail. For example, on a printing press, anything with less ink than 3% Cyan, 2% Magenta and Yellow does not print at all, it just comes out as a blank paper. The minimum density that holds good highlight detail can be described a close to 7%C, 5%M, 5%Y.
On a continuous-tone LightJet printer any detail above RGB values of 245, 245, 245 will be barely visible, and the shadows below 7,7,7 look as solid as blacks. On the other hand, a typical Epson printer - being a halftone device - can hold some visible detail even at 250,250,250; or rather you can see some ink dots sprinkled around.
Therefore adjustments that need to be made depend on what printing process the image is going to end up with.

OPTIMIZING CONTRAST

Maximizing the output range with white and black points is only one half of the story - in order to look its best, the image contrast needs to be adjusted so the available tonal range is utilized in the best possible way.
For the most part this means finding a compromise between getting maximum contrast on a main subject of a picture and plugging up shadows and blowing out highlights.

Lets take a look at this example.

This is the original image, the way it was captured. Nothing wrong with this whatsoever.
Let's try something, though!
Mouse over to see the image with optimized white and black points, as discussed in a previous paragraph.
Notice that I made a conscious decision to be very aggressive with my adjustments - most of detail in the background is gone, and the shadows are clipped severely in the area where headphone meets woman's hair.

Is this reasonable? For me - yes.
The parts of a tonal range that got lost did not mean anything to me. I don't really care whether you can see clearly that piece of office machinery behind woman's hand, or what shape is the headphone cup.
What I won in this bargain is increased contrast in the areas that do interest me - skin tones. The same logic will be applied to a contrast adjustment - we lose some (not so important details) and we win some (increased range and definition on a main object of interest).

This final step is the image with contrast adjustment. You can see that shadows got compressed further and highlights are even more pronounced, making for a very "alive" image. Is this too much? Maybe, if your primary objective is to make this look good on screen.
This image is going to a printing press though, so we know that it will flatten down somewhat. How much will it flatten down? Let's find out! - in the next paragraph.

SOFT-PROOFING

Soft-proofing part of a colour management system allows you to preview on-screen what the printed image is going to look like.
Here it is.

Notice the strong yellowish cast in highlights - it comes from the fact that this particular output profile is built for run-of-the-mill coated press stock.
In reality, when looking at the actual print, the cast does not come out quite as strong, - our eyes adjust quickly - so lets disregard that for time being.
However, overall flattening of the tones, weak shadows and subdued colours are very much for real. Having looked at the soft-proof I'm going to do one final adjustment - make it just a touch lighter.

Below is my final image, shown here with and without soft-proof on. To make things even more dramatic, underneath it I'm including the original shot along with its soft proof - just mouse over to see them.

8 OR 16 BIT?

Over the past 5 years a significant amount of effort was spent on developing tools for true 16bit workflow. The newest versions of Adobe® Photoshop® brought up to speed most of filters and colour-correction tools so, if you wanted to, you could probably switch your entire workflow to 16bit processing.

Combined with ever-faster computers, better screens and huge hard drives, this makes a fully 16bit workflow an easily achievable goal.
But is it worth it? Aside from noticeable file size and processing overhead, there's also an issue of versioning and archiving, because most popular formats for saving backups and submitting files for printing do not allow 16bit files.

This will be the question you'll have to decide for yourself - and the deciding factor, to the majority of photographers, artists and graphic professionals is this:
How much of an improvement in visual quality can one expect from switching to 16bit workflow?

Here's a simple checklist I've compiled based on my experience in working with 16bit images:

• 16bit precision only makes sense when the original files come from a device/process that actually produces this much tonal resolution. In my experience, the majority of modern digital equipment does not produce 16 or even 12bit of useful data in the tonal range which could benefit the most from extra precision - shadows and highlights.
  Improved precision in mid-tones is sure nice, but rarely turns out to be all that important in regards to a final printed piece.


• 16bit works wonderfully with computer-rendered images. New 3D capabilities in Photoshop CS3 Extended make this particular feature much more relevant than before.


• 16bit offers immediate and noticeable improvements to B&W image editing.


• Consider using 16bit if you are making major colour transformations in L*a*b* colour mode. I find noticeable improvements in rendition and smoothness of tones in 16bit vs. 8bit L*a*b* editing.


• While 16bit histogram looks nice(r), it's often hard to see any difference between 8bit and 16bit on-screen. That is because most videocards and screens are 8bit devices in a first place and cannot adequately display such minute differences in tone. Consider that most printers are worse than displays when it comes to colour reproduction.

To sum this up - test your camera (or scanner) both in 16bit and 8bit modes. See how much difference you get in most critical areas - highlights and especially shadows; that should be your litmus test. Do use 16bit if you work with B&W images and computer-rendered art.

WHICH COLOUR SPACE TO USE

There are two basic approaches when selecting a working colour space for your capture and editing workflow:

• Sticking early on to a colour space of your output device or, better still, - to a synthetic colour space with a gamut roughly equal to that of your output device. This approach is usually called "early binding". One example is using sRGB colour profile as your working space from scanning/capture onward. sRGB is popular, widespread, contains a limited colour gamut and, generally speaking, easy to print on a variety of output devices.


• Sticking to a wide gamut, image-specific working colour space, such as ektaRGB (colour space designed to contain all the colour gamut of Ektachrome film), making all your edits in there, and finally converting to a destination colour profile for printing.
  This is "late binding". It allows you to stay with the wide gamut of your capture device throughout the editing stage, making re-purposing somewhat easier, as well as keeping your image more future-proof.

From the standpoint of practicality, predictability and certain "foolproof-ness" I have to recommend the early binding approach - at least for projects which are intended for a particular media output (i.e. photo or giclée project needs to be treated differently than the stuff that gets projected).

Happy optimizing!

Dimitri,
Colourgenics staff

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