Measurement Time and Settings

How long does it take to acquire a spectrum, from measurement start to end of data transfer from sensor to computer storage? Here we provide answers that relate to our NIRONE Sx.x sensors. For clarity of the calculation, consider three parameters:

  • Single Spectrum Completion Time — The time taken to actually measure a spectrum by the sensor.

  • Lamp Mode — The Lamp Mode defines the phases, which must be performed during the measurement, and will affect the measurement time. It might be easiest to compare the differences in measurement times based on the Lamp Mode. The three lamp modes available are Manual, Auto Lamp, and Auto Lamp with Dark Substraction.

  • Total Measurement Time — The time, which will combined into a single data entry, and the transmission of the data file from the sensor to the measurement computer. The time takes into account the repeated spectrum measurements performed.

The time taken to measure a single, Point Averaged spectrum only, is:


Single Spectrum Completion Time = T(SSC) = Number of Wavelength Points * [Wavelength Switch Time + ( Point Average * 0.025 ms )] + Wavelength Switch time
    

Manual Mode

The Total Measurement Time can be estimated with the following variables:


Total Measurement Time (Manual Mode) = Scan Average * T(SSC) + Data Transfer Time + Any Other Delays In Command Execution
      

Continuous cycle simplified:

  1. Measure spectrum
  2. Data transfer
  3. Repeat

Auto Lamp Mode

The Total Measurement Time can be estimated with the following variables:


Total Measurement Time (Auto Lamp) = 2 * Switch Lamp + Scan Average * T(SSC) + Data Transfer Time + Any Other Delays In Command Execution
      

Switch Lamp Time is 500 ms, and it is applied twice because the lamp is switched on at the beginning of the cycle and it is turned off at the end.

Continuous cycle simplified:

  1. Switch lamp on, stabilize
  2. Measure spectrum
  3. Switch lamp off
  4. Data transfer
  5. Repeat

Auto Lamp with Dark Substraction Mode

The Total Measurement Time can be estimated with the following variables:


Total Measurement Time (Auto Lamp with Dark Substraction) = 3 * Switch Lamp Time +  2 * Scan Average * T(SSC) + Data Transfer Time + Any Other Delays In Command Execution
      

Continuous cycle simplified:

  1. Switch lamp on, stabilize
  2. Measure dark
  3. Switch lamp on, stabilize
  4. Measure Spectrum
  5. Switch lamp off
  6. Data transfer
  7. Repeat

Default and Measured Values

The default Wavelength Switch Time is currently 1 ms, which is the minimum time taken. The switch time is time during which the sensor sets a new wavelength, and it includes the time for wavelength settling.

The Data Transfer for a single 512 wavelength spectrum with a Point Average of 1 has been measured to be less than 100 ms, or about 83 milliseconds at the moment. On slower systems and in the event of increased demand of resources on the measurement computer this might might be increased (the Other Delays parameter).

Example calculation

Measurement settings:

  • 201 wavelength points
  • Point Average = 100
  • Scan Average = 1
  • Averaging Mode On

Time taken to measure a single spectrum, or T(SSC), is:


T(SSC)  = 201 * [ 1 ms * + (100 * 0.025 ms)] + 1 ms = 704.5 ms = 0.7045 seconds
      

In Manual Mode, the Total Measurement Time is approximately:


T(TM_Manual) = 1 * 704.5 ms + 83 ms = 787.5 ms, or approximately 0.79 seconds
      

In Auto Lamp Mode, the Total Measurement Time is approximately:


T(TM_AutoLamp) = 2 * 500 ms + 1 * 704.5 ms + 83 ms = 1787.5 ms, or approximately 1.79 seconds
      

If Auto Lamp with Dark Substraction would be used instead, the time taken to measure the same spectrum would be:


T(TM_DarkSub) = 3 * 500 ms + 2 * 1 * 704.5 ms + 83 ms = 2992 ms, or approximately 3.0 seconds
      

Measurement Settings in SensorControl

You can view and modify measurement settings in the Settings tab of SensorControl software. Define the wavelength vector before starting the measurements, either automatically or manually. The wavelength vector may have a maximum of 512 points to it, that is, you can set 512 different wavelengths to be measured. The minimum wavelength step is 0.1 nm. You can set the wavelength vector in two ways:

  • Full spectrum
  • Manual selection

The wavelength points must be available for the sensor to be measured, that is, the software only accepts values within the sensor range.

Full Spectrum

This is the default mode for the sensors. In this mode, you can set the first and last wavelength points that you want to measure, and a general step with which new points will be automatically calculated into the spectrum. The calculation of points starts with the first wavelength and ends on the last wavelength point, unless it cannot be included in the measurement. This may happen if the step different between the penultimate and the last wavelength point is less than the step size, or if the wavelength vector has become too long.

For example, the settings below will result in a wavelength vector with 26 data points, starting from 1600, 1610, 1620, 1630, ... 1830, 1840, and 1850. Neither 1855 nor 1860 will be input into the vector.

  • First wavelength = 1600 nm

  • Last wavelength = 1855 nm

  • Step = 10 nm

Manual Selection

Manual selection is a manual vector input mode. In this mode, wavelength points can be input into the vector manually, and they will be measured in exactly the listen listed order.

The special use case for this mode is that the same wavelength points can be input multiple times into the same wavelength vector, and they will be separately measured and saved to different columns in the save data text files. This is especially useful when measuring moving objects.