The Real Reason One-Shot Color Images Look Green: Quantum Efficiency, Not the Bayer Ratio

When shooting astrophotography, we often notice that the Raw files from an OSC (one-shot color camera), or the linear image after integration, show a pronounced green color. In past discussions of this phenomenon, the most common explanation has been: “Green pixels make up 50% of the Bayer Array, twice as many as the red and blue pixels, so it’s naturally greener.”

However, according to a viewpoint recently shared by Seti Astro, this explanation isn’t rigorous in terms of signal-processing logic.

The Bayer Ratio Boosts SNR, Not Average Brightness

In fact, the reason an image looks green has no direct relationship with the ratio of pixel counts.

In the debayering algorithm, the software fills in the color information through the average of surrounding pixels, rather than summing the values. This means: having twice as many green pixels boosts the green channel’s signal-to-noise ratio (SNR) and spatial resolution, making the image finer; but it does not double that channel’s “average brightness”.

In other words, the doubled green pixels in the Bayer Array are a quality benefit—not the reason the image ends up with a green cast.

The Real Reason: The Sensor’s Difference in Quantum Efficiency

The real core lies in the sensor’s difference in “quantum efficiency (QE)”.

You can see from the spectral response curve that most silicon-based sensors are far more sensitive to the green band than to red and blue light. So under the same exposure conditions, the electronic signal (ADU value) produced by green pixels is inherently much stronger than the other channels.

On top of that, astronomical raw files, in order to remain scientific, usually don’t apply white-balance calibration weights in advance—unlike ordinary daytime cameras, which balance it in-camera first—so this physical sensitivity bias is preserved intact, which then causes the visually green cast.

Understanding this actually helps a lot with post-processing: the green cast isn’t the camera being broken, nor an “inherent defect” of the Bayer Array, but a normal phenomenon of color-calibration weights not yet being applied. Once you finish color calibration at the linear stage, this layer of green naturally gets balanced back out.