This is a common use of the term 'linear' within the film and tv business, but is actually incorrect.

TV gamma is an adjustment applied to an image to overcome the fact display CRT's (televisions) don’t produce a light intensity output that is proportional to the input signal voltage, with 50% input signal giving only 18% intensity output. As camera systems produce output voltages proportional to scene intensity (linear) a gamma correction is required when displaying the image…

The Gamma correction value is approx 2.5, but does vary from system to system, with Macintosh computers being the notable different system with a Gamma correction of 1.8, and is based on printers, not CRT displays...

The NTSC tv system actually specifies a gamma of 1/2.2 = 0.45, while the PAL tv system specifies 1/2.8 = 0.36.

The gamma correction value is also not a 100% inverse of the CRT gamma value as it also compensates for the apparent reduction of contrast which occurs when a TV is viewed in a typically living room environment.

This is much like the ’enhanced’ contrast applied by a film print image, which has a gamma of 3 to OCN's 0.6, giving a combined gamma of 1.8 rather than the expected 1.

The following images show the difference between a true 'linear' image, and the same image 'gamma corrected'.

Linear image

Gamma Corrected

Notice how the 'whites' and 'blacks' are not affected by the gamma correction.

However, a 1 bit display that produces grey scales by dithering between two values will have a linear intensity response no matter what the monitor's gamma response is. This fact can be used to demonstrate the effect of display gamma further with the use of the following images...

The image contains 2 rows of 3 squares, with the value of the squares in each row varying from 25% to 75%. The top row uses grey values; the bottom row simulates the greys by dithering. On a display which corrects for monitor gamma the top squares will have roughly the same apparent brightness as the corresponding bottom squares. On a system which does not correct for monitor gamma the top row of squares will appear much darker than the bottom row. Standing about 6 feet from the monitor and squinting a bit gives the best results!

The image below allows you to directly estimate the gamma of your display system. Stand about 6 feet away (squint!) and decide which column of the image comes closest to having equal brightness in the top and bottom halves. The number under this column is the gamma of your display system.

The above grey scale images and associated information has been extracted from Robert Bergers website with thanks.

The real world images within the Light Illusion website are set for a display gamma of 2.2, as this is the world-wide standard for images on the Web, and Microsoft...