The BC20 has a peak measurement and hold circuit which measures the peak power on the detector and holds it. Therefore when a beam is scanned over the detector, when the beam is on the detector it goes up to a peak which corresponds to the same power the detector would measure if the beam was stationary and therefore the BC20 reads the correct power whether the beam is scanned or not. In order for the BC20 to do this, the beam must be on the detector (of size 10x10mm) for at least ~13µs and therefore this limits the scanning speed on the detector to 30,000 inch/s.
Power Meters FAQ's
Special Photodiode Sensors
The Ophir integrating sphere sensors, models 3A-IS and 3A-IS-IRG have a white diffuse reflecting coating on the inside of the integrating sphere. The sensitivity of the sensor is quite sensitive to the reflectivity of the coating. If the coating absorption goes up 1%, it can cause a 5% change in reading. Therefore, care must be taken not to soil or damage the white coating of the sensors. Also it may be a good idea to send the sensors for recalibration yearly.
Integrating Sphere Theory
Integrating spheres are used when we have divergent light sources. As shown in the illustration, an integrating sphere has its inner surface coated with a surface that highly reflects (typically 99%) in a scattering, nonspecular way. Thus when a divergent beam hits the walls of the integrating sphere, the light is reflected and scattered many times until the light hitting any place on the walls of the sphere has the same intensity.
A detector placed in the sphere thus gets the same intensity as anywhere else and the power the detector detects is thus proportional to the total incident power independent of the beam divergence. (The detector is so arranged that it only sees scattered light and not the incident beam). An ideal integrating sphere has a surface with reflective properties are Lambertian. This means that light incident on the surface is scattered uniformly in all directions in the 2pi steradians solid angle above the surface. The surface used by Ophir closely approximates a Lambertian surface.
The 3A-IS series has two 50mm integrating spheres in series with a photodiode detector. The two series spheres scramble up the light very well thus giving output very independent of incident beam divergence angle. The two spheres in series also insure that the light hitting the detector is greatly reduced in intensity thus allowing use up to 3 Watts even though photodiodes saturate at about 1mW. There are two models, the 3A-IS with a silicon photodiode for 400 – 1100nm and the 3A-ISIRG with an InGaAs detector for 800 – 1700nm
The auxiliary LED is used to increase accuracy in cases where the source under measurement is reflective or protrudes into the integrating sphere. The auxiliary LED is used to measure this effect and calibrate it out.
The auxiliary LED emits at 390nm, while the measured UV LED can emit between 350nm and 400nm. Will this increase the measurement error?
For the most demanding accuracy requirements, a broadband source is used for the auxiliary lamp, and a spectrometer monitors the effect of self-absorption across the spectrum. For UVLEDs, in the limited spectral range of 350nm-400nm, using the auxiliary LED at 390nm is an efficient solution, and the error due to self-absorption is reduced from up to ±20% to up to ±5%.
I am measuring a 375nm LED. The auxiliary LED is emitting at 390nm. Do I need to set the wavelength at 390nm when I am making the self-absorption calibration measurements?
No. the self-absorption measurement is a relative measurement, so keep the wavelength setting at wavelength of the LED you are measuring.
I wish to increase the accuracy of the measurement. Can I get the auxiliary LED in a different wavelength than 390nm?
This is easy with the PD300-MS all you need to do is place a drop of oil on the center of the sensor surface and immerse the objective lens in it. After measurement just wipe down the sensor and clean it for another use.
The sensor is large in order to 'catch' light coming out of the objective at large angles, it is most accurate when the light is centered on the surface – you should use the target on the backside of the sensor to position the sensor optimally.