EMVA Data

The EMVA data key gives information about the sensor performance in relation with the EMVA 1288 model standard. There are 9 sub-keys to fill:
  • IR & UV Filter
  • Quantum Efficiency
  • Bayer Matrix
  • System Gain
  • Temporal Dark Noise
  • AST
  • DR
  • Dark Current
  • Spatial Non Uniformity

IR & UV Filter

A UV and/IR cut can be added on the sensor cover window. If present, the IR & UV Filter key allows you to take this filter into account.

The value taken is a link to a *.spectrum file (Speos native format).

Note: Setting the value to None define the filter as not present (or already specified into the Speos Camera sensor definition).

By default, *.spectrum files are expressed in percentage. In this case Unit key must be Percentage.

Note: You can also define the Unit sub-key in [0;1] if the data in the *.spectrum file correspond to that.
   IR & UV Filter: 
      Filename: c:/uv_ir.spectrum  # None or link to a *.spectrum file
      Unit:  # 'Percentage' or 'O to 1'

Quantum Efficiency

The Quantum Efficiency key value is a link to a *.spectrum file that represents the sensor quantum efficiency.

Usually, data given by sensor manufacturers are the combination of the sensor efficiency and the RGB Bayer filter. In this case, the value should be a *.spectrum file with 100% constant value.

   Quantum Efficiency: #(nu)
      Filename: c:/QE.spectrum  # link to a *.spectrum file
      Unit:  # 'Percentage' or 'O to 1'

Bayer Matrix

Monochrome Sensor

In case of a monochrome sensor, The Bayer Matrix key is not used and the Type sub-key value must be set to None.

Bayer Matrix:
Type: None

RGB Sensor

In case of a RGB sensor:
  • the Type sub-key value must be a combination of 4 letters chosen in ‘R’, ‘G’ and ‘B’.
  • The following four sub-keys 00 spectrum, 01 spectrum, 10 spectrum, 11 spectrum must be set with a link to a *.spectrum file.
   Bayer Matrix:
      Type: RGGB
      00 spectrum: c:/Red.spectrum   # link to *.spectrum file
      01 spectrum: c:/Green1.spectrum   # link to *.spectrum file
      10 spectrum: c:/Green2.spectrum   # link to *.spectrum file
      11 spectrum: c:/Blue.spectrum    # link to *.spectrum file

System Gain

In the sensor, the charge units accumulated by the photo irradiance is converted into a voltage, amplified, and finally converted into a digital signal by an analog-to-digital converter (ADC). The whole process is assumed to be linear and can be described by a single quantity, the System Gain key.

The System Gain units is DN/e- (digits per electrons).

   System Gain:
      Value: 0.5
      Unit: DN/electron

Temporal Dark Noise

Dark Noise results from the fact that even if there is no light at all hitting a pixel, the photodiode "faucet" still has a small flow of "leakage" electrons that are generated thermally.

The Temporal Dark Noise value can be expressed in electrons, bits or dB.

   Temporal Dark Noise:
      Value: 3
      Unit: Delectron

AST

Absolute sensitivity threshold (AST) is the number of photons needed to get a signal equivalent to the noise observed by the sensor.

The Value sub-key must be a positive number in electrons or in photons Unit.

In case of photon-based value, you must define the Wavelength sub-key corresponding to the wavelength of the photon (to be able to convert into electron using Gain system). Wavelength unit is nm.

   AST: # Absolute sensitivity threshold
      Value: 3
      Unit: photon   # 'electron' or 'photon'
      Wavelength: 545  # in nm (used only in case of given unit in photon)

DR

Dynamic range (DR) is defined as the ratio of the signal saturation to the Absolute Sensitivity Threshold (AST).

The Value sub-key is a positive number in DN, bits or dB Unit.

   DR: # Dynamic Range
      Value: 3500
      Unit: DN

Dark Current

The dark signal is mainly caused by thermally induced electrons. Therefore, the dark signal has an offset (value at zero exposure time) and increases linearly with the exposure time. Because of the thermal generation of charge units, the dark current increases roughly exponentially with the temperature.

The Dark Current key is based on four sub-keys:
  • the Mean sub-key corresponds to the average value of e-/s for the Tref temperature.
  • the Standard Variation sub-key corresponds to the variation around the mean value in e-/s.
  • the Tref sub-key corresponds to the value for Reference Temperature.
  • the Td sub-key corresponds to the temperature interval that doubles the dark current.
   Dark Current:
      Mean:
         Value: 8
         Unit: electron/s    # 'electron/s'
      Standard Variation:
         Value: 4
         Unit: electron/s    # 'electron/s'
      Td:                    # Doubling Temperature Interval
         Value:  10
         Unit: K             # 'K' or 'C'
      Tref:                  # reference temperature
         Value: 20
         Unit: C             # 'K' or 'C'

Spatial Non Uniformity

The model discussed so far considered only a single or average pixel. All parameters of an array of pixels will however vary from pixel to pixel.

For a linear sensor, there are only two basic non-uniformities. The characteristic curve can have a different offset and different slope for each pixel.

  • The dark signal varying from pixel to pixel is called dark signal non-uniformity, abbreviated to DSNU.

    DSNU value is defined in DN.

  • The variation of the sensitivity is called photo response nonuniformity, abbreviated to PRNU.

    PRNU value is defined in percentage.

   Spatial Non Uniformity:
      DSNU:
         Value: 3
         Unit: DN    # 'DN'
      PRNU:
         Value: 5
         Unit: percentage