So what is the color balance of a digital sensor? The short answer is that, unlike film stock, which comes in either daylight or tungsten balance, camera sensors don’t have a “color balance.” Yes, all cameras allow you to choose a preferred color temperature or to automati-cally “white balance” to adjust to daylight, tungsten, overcast, fluo-rescent, etc, but these are electronic corrections to the signal — they don’t actually affect the sensor itself.
On some cameras, this color correction is “baked in” and is a perma-nently part of the image. As we know, cameras that shoot RAW may display the image with corrected color balance, but in fact, this is just the metadata affecting the display — the RAW image remains unaf-fected and any desired color balance can be selected later in software and only needs to be baked in at a later time (there are some excep-tions to this, as we’ll see). However, that being said, some camera sensors do have a color balance that they are optimized for — this is called their “native” color balance, just as their “built in” sensitivity is referred to as their native ISO. Some are known to have slightly different responses at various color temperatures. What is the native ISO or white balance of a sensor is generally determined by what setting requires the least amount of gain (electronic amplification) to be applied. This results in the least noise.
For example, the native color balance for the Dragon sensor used in some RED cameras is 5,000 degrees Kelvin, but of course can be electronically compensated for any color temperature in the range 1,700 to 10,000 Kelvin — as we know, this is recorded in the metadata when shooting RAW images. White Balance presets are available for Tungsten (3200K) and Daylight (5600K) lighting; the camera can also calculate a color neutral White Balance value using a standard white card/gray card technique.
Canon takes the approach that color balance is baked in by applying gain at the time of shooting; see the chapter Codecs & Formats for a discussion of their logic in choosing this path.
CANON’S APPROACHTO SENSORS
Canon uses a color filter arrangement but the sensor is actually out-putting essentially four 2K streams, one for the red pixels, one for the blue pixels, and two streams for green pixels — Gr and Gb. The second green signal is placed in the signal where the alpha channel (transparency or opacity) would normally go in other implemen-tations. These signals are then multiplexed together to form RMF (Raw Media Format) files. (Figure 1.19).
ALEXA NATIVE COLOR TEMPERATURE
The following is from Arri and is used with their permission. “What is the native color temperature of Alexa’s sensor? The short answer is that while Alexa does not really have a ‘native’ color temperature, the
Normal 4K Sensor Sony 8K Sensor Figure 1.17. (Right) Sony’s
arrange-ment of receptors allows for denser packing of the photosites. (Courtesy of Sony).
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point at which the lowest overall gain is applied to the red, green and blue pixels is at 5600 degrees Kelvin, yielding the lowest possible noise in the image. However, since Alexa has an amazingly low noise level anyway, the difference in noise between 3200K and 5600K is so minimal as to not be relevant in most shooting situations. So choos-ing the color temperature can be dictated by other factors, such as the cinematographer’s preference or the availability and/or cost of tungsten or daylight lighting instruments.
“For the long answer, we have to start with the birds and the bees, or in our case, with celluloid and silver halide crystals. Film stocks are balanced for either a tungsten (3200 degrees Kelvin) or a daylight (5600 degrees Kelvin) light source. To achieve this, film manufac-turers carefully tune the chemistry of the individual color layers. A grey card filmed under the respective lighting conditions should also result in a grey image after development. Thus each film stock has a given color temperature ‘baked-in,’ which is sometimes also called the ‘native’ color temperature of that film stock. If you need a differ-ent color temperature, you change film stocks.
“The way light is converted to an image is different for film and sensors. In order to display a grey card as grey, digital cameras have to carefully balance the gain (amplification) applied to the red, green and blue (RGB) signals. The response of a digital camera to incoming light of different colors is determined by the response behavior of the filter pack (IR, OLPF, UV), the photocell, the Bayer mask inks and the image processing. Even though the properties of the filter pack, photocell and Bayer mask inks are chosen with the best color balance in mind, there are other factors that also influence the color balance of the signal coming from the sensor, including an optimiza-tion for highest sensitivity, widest dynamic range and lowest noise.
The proper balance between all those requirements is not only
dif-Figure 1.18. (Above) The Sony F-65 at a camera checkout. The recording module is mounted on the back and a TVLogic monitor is set up for the camera assistant. (Courtesy of DIT Sean Sweeney).
ficult to achieve, but also one of the factors that differentiates the various models of digital cameras.
“Since one can neither create a separate sensor for each color tem-perature situation, nor readily change them if one could, digital cam-eras have to cover a variety of color temperatures with one sensor.
For any given color temperature, a sensor will deliver an unequal amount of red (R), green (G) and blue (B) signal. In order to bal-ance the three colors for different color temperatures, digital camera manufacturers use different amplification settings for red and blue, while keeping green un-amplified. Let’s look at the actual settings the Arri Alexa uses to illustrate this.
“When using Alexa to capture a grey card lit with a tungsten source (3200K), the signals from the red and blue pixels have to be amplified by the following amounts to make a neutral grey:
R — 1.13x G — 1.00x B — 2.07x
“Shooting the same chart, but now lit with a daylight source (5600K), will put more blue and less red light on the sensor. So we can apply less amplification to the signal from the blue pixels but need a little more amplification for the signal from the red pixels:
Figure 1.19. Canon’s sensors output red and blue channels, and also two different green channels. These are multiplexed into a 4K RAW image which can then be processed into video. RAW data is not really view-able, so it is not considered to be
“video.” Also note that Canon’s pro-cess applies ISO at White Balance early in the process, unlike most other cameras. (Diagram courtesy of Canon).
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R — 1.64x G — 1.00x B — 1.37x
“So, for tungsten, Alexa uses a little more amplification in the blue channel, and for daylight a little more in the red. And for those who are still reading and want more: even though the red and blue ampli-fications are equal at 5000K, daylight (5600K) has the mathemati-cally lowest overall noise gain, measured as the square root of the sum of the squared gains.”