Phototherapy meter calibration

Optical radiation has been demonstrated to have a number of therapeutic photobiological effects. The most efficacious spectral range is approximately 400 nm to 500 nm, being from the near ultraviolet to the upper edge of the blue end of the visible spectrum. Conditions such as jaundice and infant hyperbilirubinemia benefit from phototherapy treatment to regulate bilirubin levels. The same region of the spectrum presents potential hazards at high irradiance levels. To reduce hazards & optimise benefits, phototherapy meters have been designed to measure in the active spectral range of phototherapy irradiators to monitor dosage.

Calibration of phototherapy & bilirubin meters

Phototherapy meters measure irradiance, the units of measurement are typically uW/cm^2/nm, although newer meters on the market are moving towards using uW/cm^2. Although there is a nominally defined skin absorbance curve to promote beneficial effects, there has been no historical agreement between manufacturers on the shape of the spectral response curve of meters. This means that meters from different manufacturers will display (sometimes vastly) different readings for the same phototherapy irradiator. By way of comparison, Table 1 below contrasts the manufacturer specified characteristics of some commonly encountered meters.

LightLab calibrations are meter agnostic and comprise two stages. Stage 1 uses one or more narrow or wide band sources to characterise the spectral response of the meter. Stage 2 uses a 450 nm narrow band source to calibrate the linearity of the detector; range of calibration up to 300 uW/cm^2/nm (corresponding to 30 mW/cm^2).

Meter	Characteristics
International Light, ILT74	Approx gaussian response, 425 to 475 nm, 450 nm peak
Olympic, Bili-Meter Model 22	Tophat response, 425 to 475 nm
Dale Technology, Dale40	429 to 473 nm, peak response at 453 nm
Minolta / Air-Shields / Vickers / Narco Scientific, Fluoro-Lite Meter 451	Meter range: 400 nm to 520 nm, Meter bandwidth 60 nm, Centre wavelength 450 nm
Philips, Joey JD100	425 to 475 nm, 450 nm peak
Ohmeda Medical, BiliBlanket Meter II	Meter range: 400 nm to 520 nm, Meter bandwidth 60 nm, Centre wavelength 450 nm

Table 1 : Phototherapy meters, manufacturer specifications