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. For this reason, the standard has a modular design and consists of mandatory and optional measurements. The following groups are defined in version 3.0 of the standard:
- Sensitivity, linearity and noise: mandatory
- Dark current: mandatory (temperature dependence = optional)
- Sensor non-uniformity and defect pixels: mandatory
- Spectral sensitivity: 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.
The standard can be applied to all line scan and area scan cameras and sensors. One condition is their linear behaviour, i.e., the camera signal must increase in a linear way as the light intensity increases in linear mode.
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).
|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|
Comparison of results
Finally, all results of EMVA 1288 are summarised in a datasheet or a report. The results for sensitivity, linearity and noise can be referred to for direct comparisons and the evaluation of cameras without any indepth knowledge of the standard EMVA 1288.
Measuring value Camera A Camera B Total quantum efficiency 36 % 57 % 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 dominating 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 selecting the appropriate camera for an application the basic conditions need to be specified in detail to choose and identify the important parameters