7 – How long does it take to make an image?
Three things effect on how much time it takes to make a hyperspectral image:
- the size of the image
- the integration time
- the acquisition speed of the camera
As mentioned before SPECIM hyperspectral cameras are line scan devices. Thus you need a scanner to image a full sample. That means that either the camera or the sample is moving.
To illustrate the acquisition time, let’s concentrate on the second option with the moving sample. We have a fixed camera, and the sample is on a sample tray. We place the camera at correct distance to get a relevant field of view to cover the sample completely or partly.
To get an image while keeping the aspect ratio, the pixel size along the scanning direction needs to equal the pixel size along the FOV.
Then, if the samples have a length L, the amount of lines that the camera needs to acquire to scan it completely is L / pixel_size, which is actually (L x N) / FOV.
Now that we have that covered we can concentrate on the integration time and acquisition speed of the camera, also known as its frame rate.
The integration time (T_int), is the time which it takes by the detector of the camera to collect the photons.
Then, the full time to acquire a spectral line is T_line = T_int + T_read, where T_read is the read out time of the detector. Then, for a given/chosen integration time, the maximum frame rate of the camera is 1 / T_line. If this figure is higher than the specified maximum frame rate of the camera, then the frame rate is actually the maximum specified frame rate.
Remark: a relevant integration time is set when the dynamic range of the detector is filled at 75% at the peak sensitivity of the camera when the camera is imaging a white reference tile under the used illumination.
A lower integration time can be set, but at the expense of the SNR of the system, whereas a too high integration time would saturate the detector, and the measured signal is erroneous.
Thus the scanning time is (L x N) / (FOV x frame_rate).
Now that we have got that sorted out, let’s take an example.
Scan a piece of meat (20 x 10 cm) with a RH NIR and PFD VNIR cameras. To scan the sample completely, it is wise to have a FOV of 12 cm, and to scan over 25 cm.
1) RH NIR camera: 320 spatial pixels, maximum frame rate of 350 fps.
With a proper illumination in e.g SPECIM 40 x 20 scanner, a typical integration time is 0.7 ms and the read out time is short. Thus the camera can run at 350 fps. According to the above the pixel size would be 375 µm, and the scan time (250 x 320) / (120 x 350) = 1.9 s.
2) PFD VNIR camera: 1312 spatial pixels, maximum frame rate of 65 fps.
Under the same illumination as for 1), a typical integration time with this camera is ca. 15 ms. Then the recommended maximum frame rate is about 50 fps (with a bit of margin). The image would have a pixel size of 91 µm, and the acquisition time would become 55 s.