6.2
The light beam emitted by an LED in the Ulbricht sphere is reflected
several times by the white inside wall so that the light is diffusely dis-
tributed in the sphere. Two photodiodes symmetrically positioned in
the Ulbricht sphere serve as light detectors: a sample photodetector
(D) and a reference photodetector (D
R
). The reference detector meas-
ures the intensity of this diffuse light, while the sample detector meas-
ures the intensity of the light diffusely reflected by the test area and
weakened through absorption by the indicator. The ratio of the two
intensities is proportional to the reflectance R. The electrical signals
from the detector are carried to an analog-to-digital converter (4)
which converts the analog signals into digital values. The reflectance
is calculated in the microprocessor.
The concentration or activity of the test performed is then determined
mathematically from the measured reflectance.
This is done using a test and lot-specific function stored on a mag-
netic strip on the back of each test strip and transmitted to the instru-
ment automatically when the test strip is inserted.
The general formula for the determination of concentration for sub-
strate tests is:
c = A
0
+ A
1
· R + A
2
/R + A
3
e
nR
For the line of best fit for determination of the enzyme activity:
U/l = A
0
+ A
1
· ∆C
c: concentration
R: reflectance
A
0
to A
3
: specific coefficients
n: specific exponent:
N > 0 for sigmoid curves
N < 0 for hyperbolic curves
U/l: enzyme activity in U/l
∆C: difference between 2 measured concentrations.
The respective factors for these functions were determined during
manufacture by comparison of the measurements with Reflotron Plus
with a reference method.
With the help of these functions Reflotron Plus calculates the con-
centration or activity of the desired parameter and shows this on the
display or transmits the result to a printer or to a connected computer
system.
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