FAQ - Light Measurement

NOTE: You may also find the Light Measurement Tutorial and the Light Measurement Glossary useful and here is a link to our Light Measurement Products.

 

What is the difference between foot-candles (fc) and foot-lamberts (fL) or lux and candela/m² (cd/m²)?

Is there a simple conversion from fc or lux to ft lamberts or cd/m2?

What happens to the calibration if any of the optics are removed and/or added?

What is the difference between SED and SEL?

How often do we recommend recalibration?

Should meters be recalibrated, or is it acceptable to calibrate detectors only?

If my new meter calibration says that the instrument was received by ILT out of tolerance, do I need to make adjustments to my previous readings to compensate?

What can I do if my detector over ranges?

What should I do if my detectors are coming back out of tolerance after annual calibration?

If my XRL or SSL detector cable is cut near the detector end, can it be repaired?

What temperature range does my detector work in?

How long can my detector withstand continuous exposure to UV?

How do I convert from W/cm2 to W/m2?

Does my detector require the 5-Volt Bias on?

When do I want to bias my detector?

I make flash measurements. What are the advantages or disadvantages to using the bias?

I want to make really low level measurements. Could I benefit from using the bias?

I have a thermopile-based or amplified detector. Is it safe to use the bias?

I made measurements with the bias ON by mistake. Can I trust my readings?

Do I have to have an RA (RMA) number to ship back goods for calibration?

What is the standard lead-time for service?

Can I expedite a service order?

What are your Terms and Conditions of Sale?

How do I enable Javascript in my web browser?

 

Q. What is the difference between foot-candles (fc) and foot-lamberts (fL) or lux and candela/m² (cd/m²)?
A. Fc and lux are illuminance measurements (photometric flux per unit area) typically taken using a cosine corrector or a detector with a broad field of view. Illuminance varies in inverse proportion to the square of the distance. FL and cd/m2 are luminance measurements (flux density per solid angle) typically taken with a small aperture or small viewing area. Luminance measurements are independent of distance as the sampled area increases with distance.
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Q. Is there a simple conversion from fc or lux to ft lamberts or cd/m2?
A. No, since they have different dimensions. However, the same detector and photopic filter can often be used by changing optics and the calibration factor.
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Q. What happens to the calibration if any of the optics are removed and/or added?
A. Each detector combination requires a unique calibration factor. Adding or removing optics changes the sensitivity of the combination. If the optics being used have been calibrated together you simply use the correct calibration factor. You cannot change optics without a calibration and obtain accurate readings.
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Q. What is the difference between SED and SEL?
A. These are ILT connector specifications. The SED connects to the ILT1700 and the SEL connects to our ILT1400.
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Q. How often do we recommend recalibration?
A. Annual recalibration is recommended. However, there are times, when measuring intense UV (which can more rapidly age certain filter components), that the calibration cycle is shortened to prevent out of tolerance results obtained when the calibration period is 1 year.
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Q. Should meters be recalibrated, or is it acceptable to calibrate detectors only?
A. We recommend all equipment be returned for recalibration annually.
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Q. If my new meter calibration says that the instrument was received by ILT out of tolerance, do I need to make adjustments to my previous readings to compensate?
A. Since we have no way of knowing exactly when the changes in your readings occurred, this is not a simple answer. If there were no repairs or replacement parts, your most recent reading could be adjusted to match current readings. If your calibration factor went up, you divide your previous reading by the % of change in your calibration factor. If your calibration factor went down, you multiply previous readings by the percentage of change in you calibration factor.
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Q. What can I do if my detector overranges?

A. You can move back from the source, add an attenuator and adjust calibration, or attenuate your light source.
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Q. What should I do if my detectors are coming back out of tolerance after annual calibration?
A. There are many ways to help maintain your calibration. Be sure to always cover the detector when not in use. Expose detector just long enough to take the required measurements. Shorten your calibration cycle. Add a Neutral Density Filter to attenuate the light.
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Q. If my XRL or SSL detector cable is cut near the detector end, can it be repaired?
A. No, at least 6 inches of cable is required from the detector end.
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Q. What temperature range does my detector work in?
A. 0-40°C is the standard operating range for most ILT products.
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Q. How long can my detector withstand continuous exposure to UV?
A. We do not recommend continuous exposure to UV. ILT products were to designed to take a measurement and be removed. There is no exact ratio we can provide for hours exposed and life of the detector as there are too many contributing factors. (Time, distance, heat, spectrum, age, detector type and care of detector all contribute.)
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Q. How do I convert from W/cm2 to W/m2?
A. You can multiply your readings in W/cm2 by 10,000 to obtain W/m2. To change the calibration factor so the meter reads out in W/m2, you divide the calibration factor by 10,000 and store it.
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Q. Does my detector require the 5 Volt Bias on?
A. This is the most common question we receive regarding bias. Semiconductor-based detectors (“SED”001, 007, 033, 100, as well as all SPD, XRD, and SSD model detectors) do NOT require the bias on but can, in certain applications, gain performance values in using it. Phototubes or vacuum photodiodes (“SED”185, 220, and 240) DO require the 5V bias on, which adds the 5 volts to the already present 9 volts to give a total bias of 14 volts. If you have an internally amplified detector (SED623, 624, and 625 thermopiles, or PM photo-multiplier models) then you never want to have the 5V Bias ON as it has unpredictable results (see below for more information). If you have an SED(SEL)005 GaAsP detector, using the 5V bias is possible, but not recommended due to the large amount of leakage and unpredictability in operation. If your detector needs the bias to operate or was calibrated with it ON to increase performance, it will be indicated on the calibration certificate.
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Q. When do I want to bias my detector?
A. If you have a phototube-based detector, then you always want to have it ON. If you have an unamplified, semiconductor-based detector then you only want to use to gain in increase in performance for flash measurements.
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Q. I make flash measurements. What are the advantages or disadvantages to using the bias?
A. The advantage to using the bias is the gain in performance for flash measurements. An unamplified, semiconductor-based detector gains additional performance for flash measurements in two major ways when biased. The first improvement is to speed the response time of the detector. Which basically means the output signal of the detector gets to the meter faster than if the bias were not used. The second improvement is to increase the point at which the incoming flash signal saturates the detector or impede its ability to produce a linear output to the meter. An ILT1700 and a semiconductor-based detector with a bias can generally measure a flash as fast as one microsecond. The major disadvantage is in increase in signal noise.
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Q. I want to make really low level measurements. Could I benefit from using the bias?
A. If you are using a semiconductor-based detector, the answer is no. When making low-level light measurements with a semiconductor-based detector, it is extremely important to minimize the detector’s leakage current (which these types of detectors are susceptible to). Turning the bias OFF will accomplish this. Phototube-based detectors must be operated with the bias ON, which normally results in more noise but our phototube detectors have been designed to help minimize the leakage current of the detector. Additionally, phototubes do not suffer from the same thermal noise (1/f noise) that semiconductor photodiodes experience which helps compensate for the semiconductor photodiode’s greater responsivity.
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Q. I have a thermopile-based or amplified detector. Is it safe to use the bias?
A. No. Using the 5-volt bias with any type of internally amplified detector (SED623, 624, and 625 thermopiles, or PM photo-multiplier model detectors) can have a detrimental effect on your measurements. Since these detectors are internally amplified, applying the bias voltage can actually cause the amplifier circuit to lockup and stop it’s output signal to the ILT1700. This, in turn, can have unpredictable results on the computer of the ILT1700 and can often result in display errors, such as an over-range condition with no input signal. Accidentally placing the 5-volt bias on when using one of these detectors is generally not damaging and turning the 5-volt bias and the instrument power OFF, usually fixes the problem. We do not recommend continued operation of an amplified detector with the 5-volt bias on as this could lead to damage of the ILT1700, the detector, or both.
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Q. I made measurements with the bias ON by mistake. Can I trust my readings?
A. If the detector you were using was a silicon photodiode, then the readings should be fine. For all other detector types, redo the measurements.
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Q. Do I have to have an RA (RMA) number to ship back goods for calibration?
A. Yes this is mandatory. Please refer to our RMA Information page for further details. Without proper paperwork goods would be placed in quarantine. To receive your RA (RMA) number, you can call, fax or e-mail: the model and serial number of all items (including the optics on the detector) and your PO# and they will issue you the RA.
Q. What is the standard lead-time for service?
A. 1-3 weeks is the standard lead-time for service.
Q. Can I expedite a service order?
A. Yes, 3 Day priority service for up to 2 systems or 4 total standard calibrations is available for an additional $125.00. Additional calibrations beyond the included 4 will be an extra $25.00 per calibration. No guarantee is implied, and no express charge will be incurred if we are unable to complete service on time. Excludes repairs and scanned or custom calibrations.
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Q. What are your Terms and Conditions of Sale?

A. See our Terms and Conditions of Sale.

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Q. How do I enable Javascript in my web browser?
A. Internet Explorer users should see How do I enable Javascript. Mozilla Firefox users should see Firefox Help: Options Window and scroll down to Enable JavaScript.
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