One problem is to select the important parameters from the mass of information every camera manufacturer provides with his product. This is not particularly difficult when resolution, colour format or camera interface are concerned. However, if a sensitivity comparison is to be made, direct comparison between cameras may not be possible as the way the data is provided is not standardised.
This problem was one of the main reasons for the development of the EMVA standard 1288. The target was to define a series of parameters that had to be specified for a camera or sensor and whose measuring method is clearly defined. This standard is evolving to include further standardised methods of defining specifications and parameters. The following groups are defined in version 3.1 of the standard:
|Type of measurement||Status|
|Sensitivity, temporal noise and linearity||mandatory|
|Non-uniformity, defect pixel characterisation||mandatory|
|Temperature dependence on dark current||optional|
|Spectral measurements η(λ)||optional|
Only a brief overview of the measurements and the resulting parameters can be given here. More information is available in the EMVA standard 1288 specification which can be found at http://www.emva.org.
This release of the standard covers monochrome and colour digital cameras with linear photo response characteristics. It is valid for area scan and line scan cameras.
The test layout for the measurement is also standardised. The measurement is performed without a lens installed and with a very homogeneous light source at a certain wavelength (e.g. using an integrating sphere with an LED light of 525 nm wavelength).
Comparison of results
All results of EMVA standard 1288 are summarised in a datasheet. The results for sensitivity, linearity and noise can be referred to for direct comparisons and the evaluation of cameras without any in-depth knowledge of the standard EMVA 1288.
|Measuring value||Camera A||Camera B|
|Total quantum efficiency||36 %||75 %|
|Temporal dark noise||44 e- (electrons)||10 e- (electrons)|
|Absolute sensitivity threshold||120 p~ (photons)||17 p~ (photons)|
|Saturation capacity max||25200 e- (electrons)||6900 e- (electrons)|
|Signal-to-noise ratio max||44.0 dB||38.4 dB|
It can be seen that camera B performs better where lighting is poor. A detectable signal is already generated at 17 p~ (photons). The reason is the low noise level of only 10 e-, which is the dominant noise factor in poor lighting environments, and the good quantum efficiency of 57 %.
However, the signal-to-noise ratio max of camera B is distinctly worse. In conditions with ample light, the noise of the light becomes the decisive factor. Because the value of the signal-to-noise ratio max depends directly on the maximum saturation capacity, camera A performs better in this regard.
This example shows that in order to select the appropriate camera for an application, the basic conditions need to be specified in detail to be able to choose and identify the important parameters.
The example also makes it clear that a principal statement about which camera is the better one can be rarely made. A statement about which camera is better suited to a certain application can very well be made however.