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Lightman Calibration

There are two parts to the calibration of the Lightman Sensor tester, wavelength and intensity calibration, that is X and Y axis. Wavelength calibration is checked during manufacture, every time The Lightman is used, and at annual calibration. Intensity calibration is checked during manufacture and at annual calibration.

Wavelength Calibration

Every time The Lightman is used to do an optical test on a pulse oximeter sensor, The Lightman carries out a self-calibration procedure using an internal calibration source.

This internal calibration source takes the form of a neon light source that produces emission lines across the wavelengths required, and is located within the Lightman finger. Although it is described as a neon lamp, it does in fact also contain a proportion of argon. Neon and argon vapour emits light with well-defined spectral lines in the visible and near infrared portion of the electromagnetic spectrum. These characteristic spectral lines of emission are a fundamental property of physics and therefore the wavelengths do not drift or vary.

The purpose of this self-calibration procedure is to check every time The Lightman is used whether any of the optical components within the Lightman spectrometer have been disturbed. If components of the spectrometer have been disturbed the Lightman can compensate for this, up to a point. If excessive damage has been caused, the Lightman will fail to calibrate and will not allow further use until the problem has been rectified

This self-calibration capability of The Lightman is rechecked again during the annual calibration check..

Intensity Calibration

The second part of the calibration process is the intensity correction. The spectrometer and input optics within the Lightman does not have an equal efficiency over all wavelengths, with which the device operates. This largely arises from the non-linear response of the CCD image sensor in the spectrometer.

The spectra produced by the LEDís are not discrete emission lines, but instead have an elongated sinuous shape. The efficiency of the spectra collection system (spectrometer, fibre optic, act.) can affect the shape of the output spectra, and the value of the centroid wavelength, an important figure in pulse oximetry.

To accommodate these in-efficiencies, the spectra from a light source with a known traceable intensity, the NPL lamp (National Physics Laboratory), is recorded by the Lightman spectrometer. By comparing the spectra recorded by the Lightman spectrometer and the spectra from NPL, a correction factor for each wavelength is generated and stored in each Lightman. This process is carried out for every Lightman to overcome any variation in efficiencies between devices in production and is therefore unique to each device.

Changes in the intensity correction will occur much slower than variation of the wavelength calibration, but can be identified and re-compensated for at the annual calibration check, preventing if any compromise of the operation of the Lightman. In practice Lightman intensity calibration is extremely stable, we have never seen any sign of drift.