Irradiance

Relative irradiance uses a lamp with a known color temperature to correct the shape of the spectrum but not its magnitude. Relative irradiance can determine whether there is more power at one wavelength than another, but not how much power there is in absolute terms.

Absolute spectral irradiance requires a lamp of known spectral power output to calibrate the spectrometer’s response at each pixel. This modifies the shape and magnitude of the spectrum, correcting for the instrument’s response function. The modified spectrum is given in terms of power per area per wavelength.

Learn more about absolute and relative irradiance in our FAQs section.

A modular setup for measuring LEDs comprises an Ocean Optics spectrometer like the SR2, HR4 or Ocean HDX models, plus optical fibers and an integrating sphere to collect the output from the LED. Also, we can add an LED power supply/controller to operate the LED and adjust its drive current, and for absolute irradiance, include a radiometrically calibrated VIS-NIR source designed for use with an integrating sphere.

Absolute irradiance measurements require calibration using a source with known power output. A spectrum is measured with the sampling optic (cosine corrector, integrating sphere or optical fiber) connected to the calibration light source, and is then compared to the known output power of the radiometrically calibrated light source. Here are some important tips to remember for absolute irradiance measurements:

• Before calibrating the spectrometer, warm up your light source as directed in the user’s manual before calibrating the spectrometer. This will ensure best results, as some lamps may require up to 40 minutes to become stable.
• Use the shutter to block the light for dark measurements. Avoid turning off the lamp once it’s warmed up. And here’s a pro tip: After you take a dark measurement, don’t forget to reopen the shutter to make measurements – something even the folks here occasionally forget.
• Save the calibration files that came with your radiometrically calibrated lamp. You may need them for future use.
• Make a note of the collection area of your sampling accessory. You can find this information in the user’s manual or in section below.
• If your sampling setup changes – for example, if you switch from an integrating sphere to a cosine corrector – a new calibration is required.
• If you use your radiometrically calibrated lamp frequently, consider tracking its usage. Most of our lamps will require recalibration after 50 hours of use; refer to the user’s manual for details.

Determining the Collection Area of Your Sampling Optic Absolute Irradiance Measurements

The collection area of the sampling optic is needed to calibrate a system radiometrically. There is a step in the OceanView Absolute Irradiance Wizard where this information is entered. 

Here’s how to determine the collection area:

• If an optical fiber is used, enter the fiber core diameter in microns. 
• If a discrete cosine corrector is used, enter the collection area as 0.1193985 cm² or fiber diameter as 3900 µm.
• If a CC-3-DA direct-attach cosine corrector is used, enter the collection area as 0.40018986 cm² or fiber diameter as 7140 µm.
• If an integrating sphere is used to collect light external to it, enter the port diameter in um as the fiber diameter.
• In an integrating sphere is used to enclose the sample and collect all light internally (imagine an LED), choose the integrating sphere option in OceanView software. Collection area does not apply in this scenario.