Dark current
1) Background
· Definition: Dark current is the average output value of pixel voltage present in the camera in absence of light.
· Dark current represents the baseline pixel level for our camera characterization purposes.
2) Methods
In measuring the dark current, we took photographs with the cap covering the lens of the Nikon D70, so that no light would enter the camera. Switching the camera into the manual mode, we manually control the settings of the exposure time and the f-stop. We then photographed with the lens cap on at all the possible exposure settings on the camera. We repeated this procedure three times, and then averaged the resulting values. We then downloaded the image data into the computer in order to process using MATLAB code that we designed. Our MATLAB code completed the following steps:
1. Compute the sample average for each image.
· I used the sample average algorithm, obtained from one of the EE 362 course instructors, Peter.
· We took three sets of data with the identical method that Peter describes. Then we averaged the results of the 3 trials.
SAMPLE AVERAGE:
Let Dij , i = 1, 2, K, nCols and j = 1, 2, K, nRows, be the output values of the pixels in the image captured with the Nikon D70. The sample average is:
2. Plot the sample average as a function of exposure time.
3. Identify the dark current in the least-squares sense using the MATLAB function, ‘polyfit’.
3) Results
Figure 1. Dark Current.
Figure 1 shows dark current as a function of exposure time (in seconds). One observed that the dark current value is initially 2.5 pixel values and increases to approximately 3.75 pixel values.
Figure 2. Dark Current (log scale).
Figure 2 shows the identical graph displayed in Figure 1, dark current as a function of exposure time (in seconds), though the exposure time is in a logarithmic scale in this figure.
Figure 3. Dark Current: Find the Best Fit Polynomial.
Best fit polynomial:
Y = (2.9605e-06)*X^5 – (0.00023291)*X^4 + (0.0067064)*X^3 – (0.08667)*X^2 + (0.49734)*X^1 + 2.5128
Figure 3 is a comparison of the observed dark current vs. exposure time and the function identified by the best fit polynomial MATLAB command. The best fit polynomial function is displayed at the top of the graph and clearly matches the data well.