Professional Adjustments: Display & Projector Gamma and Color Measurement

The human eye is an extremely complex organ. It doesn't just react to optical stimulation, but also responds to expectations in the sense of perception, and rapidly comes to its limit. Good examples that prove this point are optical illusions, which the famous magician, David Copperfield, uses with the greatest of ease to cheat the audience.

When it comes to color classification, even the professional cannot rely on his or her eye as, all too often, other factors come to play, such as ambient lighting and environment which falsify any adjustments. This, by the way, is the reason why color calibrated displays are best viewed in a darkened environment, since other external influences are not able to disturb the overall impression. If the color setup of the display or projector is to conform exactly to the standards, then the human eye is simply not able to cope.

Film producers do have an element of chance to approach the standard, since they have knowledge of the original. The layman can possibly tell if the color representation is not quite accurate. Generally, however, they do not trust themselves to say anything even if the color is a touch too green or red.

Color Measurements:

Luckily, professional help is at hand in the form of light and color measurement instruments. Each color's value can be correctly determined with these technical devices provided, of course, that the user has a little interpretation experience.

Assuming that the display is capable of being setup in this manner, color correction is precise enough to ensure conformance to video standards, which are defined by three international organizations: The European Broadcasting Union (EBU), in which Europe's public service broadcasting stations have pooled their resources. The Society of Motion Picture and Television Engineers (SMPTE), which is the umbrella organization for mainly manufacturers of video technology along with broadcasting stations and universities. Finally, the International Telecommunication Union, which is a United Nations organization boasting membership from some 191 countries, and involves itself more with the technical aspects of telecommunication.

In respect of the standards from these organizations, color space is of particular relevance since it defines the range of possible colors. Here, different guidelines exist to cater for the different color spaces of PAL and HDTV material. Usually, the coordinates of the color measurements are plotted on a so-called CIE chart from the International Commission on Illumination, which contains the color spectrum the device is capable of representing. The wide-spread CIE 1931 chart, so-called since it was introduced back in 1931, does not show all the parameters of transmitted light from televisions or projectors, but it has established itself as a working worldwide communications standard for color measurement.        

The Correct Sensor:

The accuracy of color measurement, and with it the calibration, depends mainly on the sensor being used. Cheap sensors are available from approx. 90 GBP, which are simple "tristimulus" equipment with three brightness sensors and pre-defined color filters. These perform particularly well on phosphor-based projectors with high brightness.

More expensive are spectral sensors, the cheapest of which cost around the 900 GBP mark. These deliver greater accuracy on a range of equipment which does not just have phosphor-based lighting, such as UHP lamps, or backlighting. However, with reduced brightness, this equipment often falters and is therefore perhaps not the best choice for absolute brightness measurement. This deficiency can be balanced out with a good Luxmeter, for example the Minolta T-10, available for approx. 900 GBP, which is used alongside the spectral sensor.

If you are seeking total perfection, then you will need a genuine Spectroradiometer with first-rate spectral resolution and high accuracy even under low brightness conditions. For its testing purposes, AV T.O.P. Messtechnik GmbH use a Minolta CS-2000, retailing for around 27,000 GBP.

Regardless of the sensor being used, measurement result analysis requires special software such as Colorfacts (www.colorvision.ch). This software enables the acquisition of all the relevant color and brightness parameters. Even blended colors such as yellow, cyan (turquoise) and magenta (lilac) are accurately fine-tuned.

The Course of Action:

Professional calibration is far more than just color measurement of the defined color space and the determination of the white point with respect to the D65 reference value. The professional will also analyze and optimize Gray Tracking and Gamma. Accurate calibration can be performed on-site at the customer's location and can easily consume half a day. It is therefore not that cheap. However, if you have invested in a good television or projector, then this investment is well worth the outlay, otherwise a great deal of potential could remain idle and the quality of the picture nowhere near where it could be. And that would be a real shame, wouldn't it?

A color sensor, such as the "Platinum" from Datacolor, can be affixed directly on the screen, so that ambient light in the room will not affect the final color calibration. A complete professional calibration set can cost somewhere in the region of 2,500 GBP. This includes the Colorfacts software along with a tristimulus sensor for color measurement.