Possible systematic errors in measurements of skin colour using the EEL reflectance spectrophotometer

Reflectance readings with the EEL Reflectance Spectrophotometer, used in many studies of human skin colour, depend on the spatial orientation of the applicator head of the instrument. Variations of over 15% of the calibration value have been observed. The present paper shows that this orientation dependence is due to the influence of gravity on the glowing spiral of the light bulb. The effect has an electrical and an optical component. It is concluded that the orientation effect will manifest as a systematic error in skin colour studies when calibration and measurement are performed at different orientations of the applicator head. The magnitude of this systematic error may be in the order of 10% and comparisons between different studies may be inaccurate.