Measuring the Power of a CW Laser


Picture of two CW Laser Power measuring Heads with Control Unit

Laser Power Meter Heads with Control Unit


Our Laser Power Meters are designed to make the accurate measurement of continuous laser power as simple as possible.   They accept wide divergence beams and give a rapid response yet still provide traceability to the UK National Standards for CW laser power measurement.

Design of Systems for Measuring CW Laser Power

Most meters for the measurement of CW laser power up to about 100W use an almost total absorption of the laser beam on a relatively massive target.  Although very simple, this approach can give some or all of the following problems:-

By using an Integrating Sphere , our design of laser power meter avoids these problems - or at least reduces them.   The sphere is coated with a robust high reflectivity diffusing paint which spreads the absorption evenly over the whole sphere.   Also the power on the detector is reduced to a level well within the linear range (See Principle of Integrating Spheres).   With correct design, the integrating sphere gives a signal which is essentially independent of beam position, beam size and beam angle.   This allows the measurement of the diverging output from an optical fibre (up to 30 degrees).

There is a BNC output socket to send the power reading to a Chart Recorder or DVM or PC.   The design of the electronics allows you to measure the mean power of a stream of short pulses.

The usual detector in our integrating sphere CW power meter is a Thin Film Thermopile .   This has a fast response time of 0.2 secs combined with an almost flat spectral response from 0.4 to 1.5m.   For the very lowest powers the thermopile is replaced by a photo-diode (Si, Ge, GaInAs as appropriate); a thumb-wheel switch then gives the appropriate wavelength correction.

For pulsed lasers, the photo-diode systems can have a BNC socket to allow viewing of the pulse shape of a pulsed laser on a 'Scope (Please request this at time of ordering).


Model Name Detector Type Spectral Range Power Ranges N.E.P. Aperture Head Size L D
Model 55 Thermopile 0.4 - 1.5m 1mW - 1W 15W 18mm 74mm 80mm
Model 69 Thermopile 0.4 - 1.5m 10mW - 10W 150W 25mm 99mm 92mm
Model 68 Thermopile 0.4 - 1.5m 0.1W - 100W 1.5mW 35mm 114mm 112mm
M29 (Si) Si Diode 0.35 - 1m Lowest 10W - 3mW 200nW 18mm 65mm 74mm
Highest 3mW - 1W 30W
M29 (Ge) Ge Diode 0.9 - 1.6m Lowest 30W - 3mW 600nW 18mm 65mm 74mm
Highest 3mW - 1W 100W
M29 (InGaAs) GaInAs 0.9 - 1.7m Lowest 10W - 3mW 600nW 18mm 65mm 74mm
Highest 3mW - 1W 100W

Power Ranges

For the photo-diode systems there are 6 positions for the range switch.   These ranges can be put anywhere between the sets given as lowest and highest.   For example 100W, 300W, 1 mW, 3mW, 10mW, 30mW.

Other power ranges may be available; please enquire.


For the Thermopile systems the NEP is given with the bandwidth switch in the normal position.   This gives 10-90% rise time of 80ms at the output BNC. (settling to within 0.5% in 0.2 secs).   The bandwidth switch gives a further factor of 2 reduction in NEP but the 10-90% response time is then 400ms (settling to within 0.5% in 1.0 sec).

Max. Power Density

There is a risk of damage to the diffuser if the power density exceeds 20W/cm2.   This limit needs to be taken seriously.   For example, with M68 receiving 100W, a Gaussian shaped beam must have a FWHM of at least 20mm and have no little ripples on its Profile.

Calibration Accuracy

Our Calibration of a Broadband System will normally have an absolute accuracy at your chosen wavelength of close to 1.6%.   This is traceable to the National Physical Laboratory at Teddington.  Details of how we achieve this accuracy are given on our web page Calibration Accuracy.   The Calibration Certificate will also give details of the small change expected at the limits of the spectral range (usually less than 6% over the range 0.4 to 1.5m).

For a Photo-diode System we can also calibrate it to 1.6% at your chosen wavelength.  For any other wavelengths you then simply set a digital switch to the new wavelength.   The sensitivity tracks the wavelength setting correctly to better than 4% over the full spectral range allowed for that type of photo diode.   All this information is given on the Calibration Certificate.

Display Unit

Analogue or Digital Readout ?

Analogue meters are normally strongly preferred because its is easier to follow a trend and they do not suggest an unjustified accuracy.   However, if you really do prefer a digital readout, we can supply one.   It fits into the same control unit shown in the picture above.

Rapid Pulses

The Display Units are designed to correctly integrate pulsed currents from the detector.   They show the mean power of rapidly pulsing lasers of any pulse duration with any repetition rate from GHz down to 10Hz.   Thus these Laser Power Meters are ideal for measurements of lasers for fiber optic or free space communication or pulsed laser therapy.  Below 10Hz the reading will appear noisy and will read low by an amount depending on the duty cycle.   This lower limit can be changed on request at the cost of speed of response.

Temperature Effects

Water Cooling

Heads M69 and M68 have fittings for water cooling.   You should use water cooling if you will regularly have exposures to more than 5W for more than about 20secs.

Head Mounting

M6 thread (= '0' BA).   This takes most of the standard optical posts.

Non Standard Systems

About half the systems we sell are non-standard and contain modifications to suit the particular requirements of our customers.   These changes do not necessarily cost any more.   Please contact us if your exact needs do not appear on this web page.