Apparatus for determining the diffuse reflectivity of a sample surface of small dimensions

Apparatus for determining the diffuse reflectivity of a test field of small dimensions of a test carrier for determining substances contained in a body fluid, comprising:

• a semiconductor light emitter for the emission of light for a light beam having a central ray to the test field;
• a measuring receiver for receiving the light diffusely reflected from the test field and for producing an electrical signal corresponding thereto;
• evaluating means coupled to said receiver including electronic circuit means for converting the receiver signal to a measurement value corresponding to the diffuse reflectivity;
• and
• a reference channel coupled to said evaluating means and using the light of the same emitter for a light beam having a central ray for the calibration of the apparatus and for substantially minimizing at least measurement errors caused by variations in the emitter, the reference channel being so constructed that the central ray of the beam used for the reference measurement coincides, upon leaving the emitter, with the central ray of the beam used for the test field illumination, the solid angles of both beams being smaller than 0.1 steradian, and said solid angles of both beams differing by a factor of not more than 10, so that the same alignment of the light leaving the semiconductor light emitter is used for the test field measurement and the reference measurement.