A new useful digital illuminance-meter with a special luminance-tubeadapter for universal applications in lighting engineering. By Peter Marx

1. Introduction (PDF 1.2 MB)

Compared with the total turnover of electrical engineering, the extent of lighting technics is constantly rising. Therefore, the exact measurement of light technical quantities such as illuminance, luminance, luminous intensity and luminous flux gains more and more importance.  The following article presents a new, reasonable, universally usable and portable digital illuminance-meter for accurate illuminance  and luminance measurements.

The measuring of luminous flux, luminous intensity and luminance is based on the measuring of illuminance by means of the photometric fundamental law. That is, why this quantity has got a special significance. A state-of-the-art illuminance-meter should  have  the following conditions.


2. Technical  requirements  on  a  state-of-the-art  illuminance-meter

- measuring of daylight  up to 200,000 lx
measuring of  indoor  lighting  up to 2,000 lx
measuring of low  illuminances, e.g. street lighting up to 20 lx
Emergency-lighting  up to 2 lx


3.  Functions of the digital luxmeter

Based on the mentioned conditions, the digital illuminance-meter „Minilux“   was developed at the University of Applied Sciences, Berlin  in cooperation with the Technical University of Zhejiang, China. Concrete, it is a newly developed, portable illuminance-meter for universal applications concerning the whole lighting technic (Fig. 1).

Fig. 1.  The MINILUX- device  with  photosensor  (silicon-cell with V(lambda)- and cosine-correction in accordance  to DIN 5032, class B)

Fig. 2 shows the block diagram of „Minilux“.  Two amplifiers with an extremely low bias-current are used  for protection against overload, when for example high pressure  sodium-xenon lamps with pulsed current supply and a high  luminous flux crest-factor are measured (Fig. 3). The first amplifier OPA 1 serves the range, the second OPA 2 works as a mean-forming active low-pass-filter.

Fig. 2.  Block circuit of the Minilux illuminance-photometer.  Measurement capacity:

 0.001 lx…..199.9 klx  (6 ranges),   luminance capacity: 0.1 cd/m2…..19.99 Mcd/m2  (6 ranges)

Fig. 3.  Time dependence of the luminous flux of a sodium-xenon  HID-lamp supplied by pulsed high-frequency current

Accurate photometry of high-pressure discharge lamps  with pulsed current supply calls for a special photocurrent amplifier, which, in accordance with the Talbot-Plateau-law, reliably estimate the linear mean value of luminous flux.

Ordinary  luxmeters  are  generally   not  designed to measure  high pulsating  light sources . Therefore, to avoid  errors, the staff of photometric laboratories should heed this recommendation before measuring such light sources. Critical evaluation and investigation of the suitability of the photometer for measurements of pulsating light is required.

The new measuring instrument avoids these disadvantages and thereby closes a gap in the market.


4. Important technical data

Fig. 4.  V(λ)- approximation of the silicon-photosensor in accordance to DIN 5032, class B


Fig. 5.  Cosine-correction of the silicon-photosensor

The instrument measures lux-values from: 1 mlx (resolution) to 199900 lx.

(Note:  1 fc = 10.764 lx   //   1 lx = 0.0929 fc)

Range                                                     Typical application

0.000 ............1.999  lx                            Emergency measurements
00.00 .....  .... 19.99  lx                            Street lighting measurements
000.0.....    ....199.9  lx                            Interior lighting measurements
0.000.............1.999  klx                          Interior lighting measurements
00.00.....  ......19.99  klx                          Daylight measurements
000.0...    ......199.9  klx                          Daylight measurements

Fig. 6.  Luminance-tubeadapter for the Minilux-photometer

The  special  luminance-tubeadapter  (Fig. 6)  has  a measuring angle of  α = 13º. The  tube-geometry is so calculated, that the relation between  luminance and illuminance is very simple:

                               L = 100 • E                E in lx, L in cd/

The procedure of luminance-measurement is as follows:

Put the luminance-tubeadapter on the photocell and hold the tube in the direction, in which you want to measure the luminance of a surface. Read the lux-value  from the minilux-display and multiply  this lux-value with the constant factor 100.  The result is the actual average luminance.

Luminance-capability:  0.1 cd/m2…..19.99 Mcd/m2 (6 ranges)

Procedure for  reflection-measurements of a diffuse surface:  First measure the illuminance on the surface. Then measure the luminance of the surface at the same area. Calculate the reflection-value with the following formula:




5.  Fields of applications

The instrument is suitable for Further applications are possible, e.g. for:


6.  Result

The new lux-meter provides a metrological basis for innovative, energy-efficient and consequently environmentally benign lighting systems. It supports systems based on the use of natural daylight  in conjunction with supplementary  artificial illuminance which is controlled in accordance  with user demand, and  which is equipped with dimmable high-frequency electronic ballast  devices,  which enables power savings of up to 70 %. The presented digital luxmeter  contributes to the achievement of this aim.


Price list



DIN 5032                    Lichtmessung, sections 1...7
P. Marx,                      New Goniophotometers for Lighting-engineering
                                    Laboratories.  CIE 24TH Session, Warsaw 1995,
                                    Proceedings Vol. 1, Part 1, page 189 – 192
Ye Guanrong,            Light-sensor for photometry. Zhejiang University, Hangzhou, China

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Am Kleinen Wannsee 12 J
D-14109 Berlin
Phone:           +49 30 4504 2315 / 2310
Fax:               +49 30 4504 2958
Phone (priv.): +49 30 805 1980