Design of a digital photometer


Kenrick Chin

A Project Report

Submitted to W. V. Prestwich

in Partial Fulfilment of the Requirements

for Physics 4D06

Department of Physics & Astronomy

McMaster University



The design of a low cost portable photometer is presented. The design focuses on ease of use, automatic range switching and serial interface capabilities. The instrument provides a digital display of luminous intensity readings from 0.1 µW/cm2 to 100,000 µW/cm2 with a measured accuracy of ±2%. The user may also choose to display readings in units of lux and footcandle, as well as µW/cm2.


Photometers or light meters are commonly used in photography for measuring luminous intensities. Photographers use handheld light meters routinely to measure light intensities. Unfortunately, the vast majority of light meters are designed for use in photography and are graduated in exposure values with a typical resolution of ±½ exposure value. This represents a change of luminous intensity of about ±50%. Industrial photometers are usually graduated in units of lux or footcandles, but never both. Digital lux meters are available which measure intensities from 1 to 100,000 lux over three ranges with typical accuracy of ±2%. However, none of the instruments surveyed(1)(2) allowed interfacing to a computer. This feature is considered highly desirable not only for computerized data collection purposes but also for applications where it is necessary to control the incident radiation. This is essential in such areas as photo-radiation therapy, light stimulation experiments and situations where the ambient lighting must be closely monitored and controlled such as galleries and green houses.

The objective of this project is to design an instrument that provides auto-ranging display in user selectable units and computer interfacing capability.


In this report the following conversions are used:

1 lumen = 0.00147058 watt

1 phot = 1 lumen/cm2 = 10000 lumen/m2

1 lux = 1 lumen/m2 = 0.0929030 footcandle = 0.147058 µW/cm2

1 footcandle = 1.58292 µW/cm2

The most commonly used devices for measuring luminous intensities are listed below.

  1. Cadmium sulfide cell
  2. Photovoltaic cell
  3. Photoconductive cell
  4. Photodiode
  5. Phototransistor
  6. Photomultiplier tube

...discuss response time, spectral response, thermal sensitivity


The benefits of a digital solution was recognized from the start. Not only does it provide precise unambiguous display of readings but it also allows for computerized manipulation, correction and transmission of data. The wide availability of low cost microcontroller units (MCU) allows for easy implementation of many desirable features in an instrument. Here is a list of features which are possible by using an MCU in the design.

The block diagram shows the essential components of the design.







Detailed circuit diagram

Parts List

Control program


(1) Cole-Palmer, 1994 Catalog.

(2) Edmund Scientific, Optics and Optical Intruments Catalog, 1999.

(3) Chin, Kenrick. Physics 4D06 Digital Systems Laboratory, McMaster University, 1998.