Special External Detectors

How to use the detectors
The Hagner Universal Photometer is supplied with a standard external detector (a light sensitive silicone diode) for the measurement of illuminance. This detector is cosine corrected and filtered to the spectral response of the average human eye (CIE standard observer). The current generated by the detector differs from other detectors in accordance with its own particular geometrical and electrical qualities. A reference figure is printed on the metal rim of each detector showing the detector's sensitivity in pA/lux (i.e., the current generated when the surface of the detector is subjected to an illuminance of 1 lux). The Hagner Universal Photometer is calibrated directly to this detector and, for normal operation of the meter, the pA/lux reference figure may be ignored.

However, when using any of the special external detectors the pA/lux reference figure of the standard external detector supplied with the photometer should be noted and used in conjunction with the pA/unit or mA/unit reference shown on the special detector being used
(e.g. pA/lux or mA/W/m?. This may be described as follows:

The standard external detector has an Absolute Sensitivity of α pA/lux. The standard detector is then replaced with a special detector on which is printed the Absolute Sensitivity of β pA/unit. The reading obtained with this special detector is γ. Correct measurement is:

α
---
β
                       
  x γ = Correct measured value        

 

Here are two typical examples:
Suppose the standard external detector supplied with a particular Hagner photometer has an Absolute Sensitivity of 70.2 pA/lux. A very low level of illuminance requires to be measured beyond the range of the standard detector. The latter may then be replaced with an extra sensitive detector, filtered to the CIE spectral response and showing an Absolute Sensitivity of say 950 pA/lux.

The extra sensitive detector is exposed to the low light level requiring measurement and the reading obtained is 15 x 0.01 = 0.15.

             

70.2
------
950

       
Correct measurement is thus          
    x 0.15 = 0.011 lux.  

For the second example, in a different situation but using the same Hagner Photometer, the requirement is to measure the variations of luminance of an exceptionally small bright surface ‑ an X‑ray image converter screen, for instance. Here the fire optic lightguide detector would be used and this detector might show an Absolute Sensitivity of 33 pA/cd/m2.
On a particular spot of the bright surface the reading obtained is 1.4 x 10 = 14.

              70.2
------
33
       
Correct measurement is thus          
    x 14 = 29.8 cd/m2  

 

It should be noted that the lightguide detector measures luminance whereas the standard detector measures illuminance. Despite of this, when using any External Detector the Luminance/Illuminance switch on the Photometer must always be switched to illuminance.

To facilitate easy and rapid measurements to be made with any of the special external detectors, the operator is advised to calculate the respective conversion factor in advance. In the first example given above the conversion factor is

70.2
------
950
                     
  = 0.074    

This would be the correction factor for each reading.

It should be noted that the models S2, S3 and S4 Hagner Universal Photometer has an output terminal for connection to an external volt meter. The signal from the output is adjustable and it is therefore a simple matter when using any of the special detectors to adjust the volt meter to direct true readings.

Model E4-X
The information given above is also valid for Hagner Digital Luxmeter, model E4-X, except that the E4-X is supplied with a SD15 as standard detector.