Tech Note 1070 Detail

Calibrate light intensity

Affected Products:

PS-2148 PASPORT Infrared Light Sensor
PS-2150 PASPORT Broad Spectrum Light Sensor
CI-6630 Broad Spectrum Light Sensor

Problem/Symptom:
How do I calibrate radiometric light intensity?

PASCO Solution:

Light intensity could refer to several distinct physical quantities, which fall into two basic categories, photometeric and radiometric.

Radiometric

  • Radiant Intensity [W·sr-1]
  • Irradiance [W·m-2]
  • Radiance [W·sr-1·m-2]

Photometric

  • Luminous Intensity [lm sr-1]
  • Luminance [lm sr-1 m-2]
  • Illuminance [lm m-2]

If one wishes to perform a photometric measurement, such as illuminance, use the PS-2177 PASPORT LUX Light Sensor, which directly measures illuminance.

If you wish to have a radiometric measurement, then use one of the appropriate thermopile sensors:

In order to measure the irradiance of an object accurately, the peak wavelength should be well within the spectral range of the sensor, which is determined from Wien's Law: Wavelength [nm] = 2.898*10^6 [nm]/[K] / Temperature [K].

None of the PASCO radiometric sensors are calibrated or have integrated diffusers. If you want to measure the irradiance of a light bulb, we recommend that you first find the total radiant power and divide by the appropriate surface area and/or solid angle to obtain the radiometric quantity of interest.

For example, irradiance [W/m^2] is the total power of electromagnetic radiation per unit area of surface. To obtain irradiance at the surface of the bulb, assuming a uniform distribution of light at the diffusive surface of the bulb, divide the total radiant power P by the surface area of the diffuser, A.

Irradiance = P / A

Note: To measure solar irradiance, we do not recommend the PS-2150 PASPORT Broad Spectrum Light because it will saturate at irradiances much less than that of the sun.

Calibration

If one wants to measure absolute light intensity, we recommend performing a two-point slope calibration:

Required

  • 1 Irradiance Standard, which typically consists of a calibrated coiled tungsten filament in an evacuated tube with a quartz envelope (to minimize IR absorption) and a regulated power supply.
  • 2 Integrating spheres

Procedure (PASCO Capstone software minor modifications required for other data acquisition applications.)

  1. Configure the system as shown above with the calibrated radiation power source at the center of the integrating sphere.
  2. Place the sensor at the output port of the second integrating sphere. (Keep it close to the exit of the sphere to avoid Lambertian angle dependence.)
  3. Select Tools/Calibration/Light Intensity/Two Standards (2 point).
  4. Turn off the irradiance / radiometric standard.
  5. When the value stabilizes, calibrate the first point.
    1. Leave the Standard Value zero.
    2. Click the "Set Current Value to Standard Value" button.
  6. Turn on the irradiance / radiometric standard.
  7. Calibrate the second point.
    1. Enter the known radiometric standard value output in the Standard Value.
    2. Click the "Set Current Value to Standard Value" button.
  8. Click Finish.

The radiometric system is now calibrated in terms of total radiation power.

To measure illuminance, use the PS-2177 PASPORT LUX Light Sensor instead.

Related Technotes:
TechNote 575 Illuminance [lux] Measurements

Creation Date: 03/5/2010
Last Modified:
Mod Summary: