Is the RED (or any other Bayer patterned) camera really 4k?

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Karthik Ganesh
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Is the RED (or any other Bayer patterned) camera really 4k?

Post by Karthik Ganesh »

Came across this article - thought it was pretty well explained...


JD Vandenberghe
Demystifying digital cinema

Is the RED (or any other Bayer patterned) camera really 4k?


The official spec of the RED camera reads as follow:

Sensor: 12 Megapixel Mysterium™

Physical Size: 24.4mm x 13.7mm (Super35mm)

Active Pixel Array: 4520 (h) x 2540 (v)

Full Pixel: Array 4900 (h) x 2580 (v)

Delivery Formats: 4K RGB

4K is a term the industry started using when scanning film stocks for digital post-production. 4k correspond to the pixel count of each lines of an image which about 4000 pixels.

The standard values for 4k goes as follow:

Standard Resolution DAR Pixels
Full Aperture 4K 4096 × 3112 1.32:1 12,746,752
Academy 4K 3656 × 2664 1.37:1 9,739,584
Digital cinema 4K 4096 × 1714 2.39:1 7,020,544
Digital cinema 4K 3996 × 2160 1.85:1 8,631,36
Of course, when in RGB color, it means each ‘pixel’ correspond to a Red, Green and Blue intensity value.

So, with the active pixel array of the RED having an aspect ratio of 1.78:1 (Super-35mm which is very close to 16:9, the HD aspect ratio) its resolution is well above any 4K standard (leaving alone the Full Aperture one with correspond to a 4:3 DAR that can be used in anamorphic cinematography for instance).

This is correct, but there is a twist.

The RED uses a sensor self-called Mysterium (now Mysterium-X) and wouldn’t release the tech spec of the chip at first. And for a good reason.

We know now that the Mysterium sensor is a Bayer pattern CMOS sensor. CMOS sensors are cheaper to produce than CCD, but have a worst signal/noise ratio (SNR) in low-light conditions. More about that in another post.

Each pixel of a CDD or CMOS sensor transform the amount of light received during exposure into an electrical level that is then quantized into a binary value. This means that those chips are only sensitive to the brightness component of light and not its chromatic value.

One way of recording color with such a chip is to use 3 sensors and two dichroic prisms assembled back to back (sometimes called a trichroic prism).

Image

As you can see on the diagram, after the light enters the prism a first dichroic mirror (F1) separates the blue component and send it to a first sensor. The second mirror (F2) separates the red component and sends it to a second sensor. What remains (green light) hits a third sensor.

A three-chip system is more expensive since you need three of them plus the prism. It takes more space, especially if you want 35mm size sensors, and is heavier. Also, you must be certain that the 3 sensors are perfectly aligned otherwise you will end up with some unwanted artifacts. Yet, the light absorption of the prism is minimal.

But the Red camera uses only one chip so it uses another technology called the Bayer filter.

Image

The Bayer filter is applied right on top of the CMOS sensor so each ‘pixel’ of the Red camera records only the intensity value of one primary color. As you can see, the Bayer filter has 2 green filters for each blue and red one. This is to mimic the human eye in which the cones sensitive to (what is almost our) pure green are as twice more sensitive to the blue are red ones. We are, then, more sensitive to green information than any of the two other primary colors (see chart below).

Image

The image coming out of a Bayer sensor contains only one intensity value per pixel. So, even if the active array of the Mysterium sensor records 11,480,800 intensity values for each image, it records only 2,870,200 red and blue values as well as 5,740,400 green values. So the red and blue information are only 30% of a 4K film scanner. The green is 60%.

To recreate a color image, it has to go through what is called a ‘de-Bayer’ process. That process interpolates the other color values of each pixel through an interpolation using its neighbor’s information.

There are different de-bayering processes with different processing loads and time. But you should always use the highest quality de-bayer process available.

Also, a typical Bayer filter single-chip image sensor absorbs at least two-thirds of the visible light with its filters, while in a three-CCD sensor the filters absorb only straight light and invisible light, and possibly a little more for color tuning, so that the three-chip sensor has better low light capabilities.

Since the Red camera uses a CMOS sensor, which has, as said above, a weaker SNR, you understand now why the Mysterium chip doesn’t perform very well in low light compare to other, yet much more expensive, professional cameras.

As a reminder, a 1080p image has 2,073,600 pixels. So, even with the Bayer filter, the color resolution of the Red is way beyond full HD. But let’s be careful when calling it a 4K camera.

Since then Red has release the Mysterium-X sensor and will release the Epic soon. The 5k new sensors will certainly compensate for its Bayer filter. It seams to offer a much lower noise in low-light.

At the end of the day, more important than the camera itself is the entire post-production chain and broadcast format of the finished film. On most shoot I have been on, the raw data where downsized to 1080×1920 before editing and would stay in that resolution until delivery.

Also, there are only handful theaters that offer 4K projection. The vast majority is in 2K and the quality seam acceptable enough.

People are often over-focused on resolution while forgetting about low-light sensitivity and color depth, which is, to me, far more important and will be discussed in another post.
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