What is gamma?

This term gamma, as you might have seen being used on tv, computer monitor, or even on a projector. You might know it is derived from the Greek language. But what is gamma and why it is used in electronics?

What is gamma?

According to Wikipedia

Gamma correction, or often simply gamma, is a nonlinear operation used to encode and decode luminance or tristimulus values in video or still image systems”

You see things with your eyes. Similarly, electronics use optical sensors to see things. The difference is sensors don’t see as how we see the world. Our dynamic range for light is much much higher. I will tell you an example. You can see the sunlight on a wall which is bright.

Along with it, you can also see dark areas like nearby houses or a bush, all at the same time. That is a huge dynamic range between the dark and bright. Your eyes have no issues seeing them. But, electronic sensors are not matured enough for this level of dynamic range.

How sensors see things?

The electronics see black as black and each double of luminance as one step higher in luminance. In other words, the camera sees light linearly. There are only limited steps where the available dark and white has to be placed accordingly. The rest is ignored or in other words, clipped. 

Also, note that We humans have the capacity to see the darker areas very detailed compared to brighter areas.

An example is, even if you squint, you cannot see much in very bright light. But in the near dark, your pupil widens and you can still see things nearby and navigate. You can even navigate with moonlight. But it is blinding to see normal led light or the sun. This explains the non-linearity perception of light by eyes. Hence sensors have to be configured according to human eye needs.

Take a look at the image by BenQ. The sensor senses linearly whereas our vision is biased to dark areas.

What is gamma?
credits: Benq

Encoding and decoding

We always want to see things on a TV or monitor closer to how it would appear in real life. Hence, each pixel of the image sensor inside the camera is tuned to see the darker side of the spectrum more than the brighter side. A curve is applied to the sensor to get this result. Tracing the curve, the pixels are tuned.

What is gamma?
Credits: https://nl.m.wikipedia.org

This curve is then decoded by your TV or monitor and restore the image to the original form. This time, it uses an inverse of the previously applied curve during the capturing stage. This makes the picture appear lifelike. This is called gamma correction or gamma. The curve used for gamma correction is called a gamma curve. 

In technical terms, Gamma can be described as Vout = Vingamma. Where Vout is the luminosity of the output picture and Vin is the luminosity of input value. This ‘gamma’ value is responsible for the curve.

What is gamma?
Credits: wikimedia

Traditionally, TVs use gamma 2.2 as it strikes a good balance between darker parts and brighter parts. Theatres use gamma 2.4. The picture might be a bit dark but it can show excellent shadow details. Without the gamma curve, we would see images brighter and less saturated. To answer what is gamma? It is a curve that is put onto the sensor to capture a lifelike picture and then the curve is inversed onto the visual device to decode the picture to get the same lifelike picture.