The Dark Side of Image Sensors

An ideal image sensor is always desired in astronomical imaging, which has negligible read noise and dark current, with almost 100% quantum efficiency. That means your SNR is only limited by your background sky noise and exposure time. The reality is, most of the time, far from being ideal.

In the last decade, the fierce competition for consumer camera market share and the investment into R&D perfected the CMOS sensor to a point far exceeding the performance of CCDs. Here let’s looked at the imperfections left in some of these famous CMOS sensors.

Recently I ported the “Dark Current Enable Tool” to the 3rd generation Nikon DSLRs. This made it possible to evaluate the image sensor more accurately and conveniently.

Uneven Bias Level

In the imaging pipeline, the first calibration step is subtraction of master bias frame. This will even up the pedestal for all the pixels before any scaling step. The master bias is usually created by combining dozens of bias frames. Now just by looking at the master bias we can check how good the image sensor and read out circuit are made.

D700-ISO200

Portion of bias frame in NC81338L (D3, D700) at ISO200

The above bias frame is from D700. Due to its use of 6 discrete AD9974 dual channel ADCs from analog readouts, the 12 column per cycle of uneven bias level is clearly visible even without frequency analysis. The Sony sensors, like the IMX028 in D3X, employed the column parallel ADC. The column-wise irregularity is much smaller due to the voltage comparators share a common ramping DAC reference. There should also be calibration circuits before readout to cancel the differences between these column circuits.

D3X

Bias pattern in D3X at ISO 1600 and ISO 100

However, I do notice a weird global pattern in the IMX028 bias frame. At first I thought it’s due to a light leak. But I quickly ruled it out since it decreases with increasing ISO and is absent in 2 Bayer channels. It cannot be a low frequency noise in power circuit either as the same pattern is observed in individual bias frames and different ISO. Could this be individual sensor issue? I don’t know. But it’s not observed in any other camera I had data access to.

Read Noise

Most calibration program only outputs a master bias frame. During this process, read noise for each pixel can be calculated by sample standard deviation without much effort. (In statistics, sample SD is a biased estimator, but none the less, consistent if sample size is large)

ReadNoise

Read noise in D700, D3X, D5100 and D800

It is clear how the sensor design had an effect on the read noise distribution. Unlike the 4T stand alone pixel design in NC81338L, all Sony CMOS inside Nikon used 2.5T shared pixel. That’s what give you the pairs of noisy pixels.

Photo Response Non-Uniformity

PRNU is very difficult to estimate unless a light source more uniform than the sensor response itself is used. Some form of PRNU, such as stitching artifact is more apparent.

Stitching artifact in IMX028

These artifacts are caused by multiexposure photolithography. Since the stepper does not have an imaging circle large enough to cover a full frame image sensor die, it has to be stitched like making a panorama. IMX028 has only one seam while the NC81338L has 3 due to a much smaller mask used.

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