IV. Detector Signal Measurement

A typical light detector or photoactive device converts impinging photons into a current or voltage proportional to the incoming signal.

The detector connects to an electronic meter for amplification, possible conversion from an analog to digital signal (ADC), calibration and display of the measurement result. Together, the meter, photodetector and accessory components form an optometer, radiometer, photometer, color, laser or optical power meter and reflection/transmission measurement systems. A radiometer consists of a voltage or current meter coupled with a radiometric type detector. Photometers employ the same meters used with photometric type detectors. Multi-channel color meters are used with colorimetric detectors to display multiple quantities. The optometer is a term used to indicate that the meter can be used with either radiometric or photometric type detector heads. Microprocessor controlled units capable of measuring currents down to tenths of picoamperes up to a few milliamperes are available. This allows full utilization of the sensitivity range of most photosensitive devices. Measurement methodology might employ 16-bit signal digitization by means of an analog to digital converter (A/D) with sampling rates in the microseconds. Selectable averaging calculation of the sampled results from microseconds to seconds provides more measurement flexibility for fast events or low-level signals. Operation of the device can be accomplished through a logical menu structure with user input via front-panel keyboard or through computer control via RS232 or IEEE computer interface.

The quantity or optical unit measured will depend on the detector type, how it’s configured in the way of filtering and input optic, and its calibration. Radiometers are available in hand-held mobile or bench-top models for laboratory use. Self-contained cordless models are used for remote dynamic monitoring where a standard detector that connects to the meter via a cable might foul. Capabilities such as dynamic measurement range, operating modes (example: CW, dose, pulse energy) and features (example: auto-ranging, backlit display, digital interface, datalogging) separate the different models. Usually the application determines what specific capabilities are important to have in the radiometer system. For example, in a UV curing production process where multiple stations must be monitored, a multi-channel radiometer with settable min/max reading feature, RS232 or IEEE interface and remote multiplexed detectors would be desirable.

The following is a list of various features, modes of operation and specifications offered in current light meters. Note that available features and functions will vary depending on the type of meter and manufacturer.

Operating Modes & Features:

CW: Continuous wave is a run of continuous type measurements. The measurement frequency depends on the integrating time and the max. sample rate. of the meter.

CW Min/Max: CW measurement where the min. or max. value that occurred during the measurement run will be displayed. The min. or max. value can be reset with the RESET switch.

CW Level Check: CW measurement where the measurement values are compared against min.-max. threshold values. The threshold values are entered into the meter by the user.

CW Level Minimium / Maximum: Menu to adjust the threshold values for CW Level Check.

Run/Hold: To freeze a measurement value on the display and stop the continuous measurement.

Relative Ratio (%): Measurement value as the relative ratio of a reference value (stored in the optometer) or a reference measurement value (2-channel optometer required).

Relative Ratio Factor: Measurement value as the relative ratio factor of a reference value (stored in the optometer) or a reference measurement value (2-channel optometer required).

Attenuation (dB or dBm): Measurement value as the logarithmic ratio factor (attenuation) of a reference value e.g. dBm (stored in the optometer) or a reference measurement value e.g. dB (2-channel optometer required).

Dose: CW measurement values integrated over the dose measurement time. A preset dose measurement time or a max. dose value will stop the measurement.

Data Logger: Each measurement value of a CW measurement will be stored individually in the optometer‘s memory. Each measurement may be manually or automatically initiated by a preset measurement cycle time. Measurement data can be outputted through computer interface.

Color: Chromaticity coordinates x,y and u‘,v‘ and the correlated color temperature are calculated from the ratio of the detector‘s signals.

Peak Maximum: Each CW measurement interval consists of a certain number of samples (number depends on integration time and sampling rate). Peak Maximum is the most positive sample of one measurement interval. A new Peak Maximum is calculated and displayed for each measurement interval.

Peak Minimum: Each CW measurement interval consists of a certain number of samples (number depends on integration time and sampling rate). Peak Minimum is the most negative sample of one measurement interval. A new Peak Minimum is calculated and displayed for each measurement interval.

Peak to Peak: Each CW measurement interval consists of a certain number of samples (number depends on integration time and sampling rate). Peak to Peak is the difference of the most positive to the most negative sample of one measurement interval. A new Peak to Peak value is calculated and displayed for each measurement interval.

I-Effective: Measures and calculates the energy of light pulses based on the Schmidt-Clausen formula. The input signal is sampled with the max. sampling rate for one measurement interval (Pulse Measurement Time). First the pulse-energy is calculated by integrating the samples. I-Effective is calculated by using the pulse-energy and the peak-value of the measurement interval using the following formula:

I-effective = peak-value * pulse-energy / (peak-value * C + pulse-energy)

C = IF-Time Constant (between 0.1s and 0.2s, depending on application)

IF Time Constant: Factor C for calculation of I-Effective (Schmidt-Clausen).

Pulse Energy: Measures and calculates the energy of light pulses. The input signal is sampled with the max. sampling rate for one measurement interval (Pulse Measurement Time). The energy is calculated by integrating these samples.

Pulse Measurement-Time: Measurement interval for I-Effective and Pulse Energy measurements.

Remote RS232: enables RS232 interface of the device. RS232 is a standard for Asynchronous Transfer between computer equipment and accessories. Data is transmitted bit by bit in a serial fashion. The RS232 standard defines the function and use of all 25 pins of a DB-25 type connector. Minimally configured, 3 pins (of a DB-9 type connector) are used, namely: Ground, Transmit Data and Receive Data. On PCs, the RS-232 ports labeled as "serial" or "asynch" and are either 9 or 25 pin male type.

Remote IEEE488: Interface IEEE488 of the device enabled. IEEE488 is a standard for Parallel Transfer between computer equipment and measurement instruments. Data is transmitted in parallel fashion (max. speed 1MByte/s). Up to 31 devices (with different addresses) can be connected to one computer system.

USB: a communication standard that supports serial data transfers between a USB host computer and USB-capable peripherals. USB specifications define a signaling rate of 12 Mbs for full-speed mode. Theoretically 127 USB-capable peripherals are allowed to be connected to one USB host computer. The connected devices may be powered by the host computer.

Auto Range: when activated, the measurement range is switched by the device automatically to the optimal value (depending on the input signal).

Manual Range: with autorange disabled, the measurement range can be manually fixed to a certain value. The device is not allowed to switch measurement ranges automatically. Manual range adjustment can be useful in cases where input signals are changing rapidly.

Calibration Factor: Optical sensors transform optical signals into current. This current is measured by the device. The calibration factor determines the relationship between the measured current and the calculated and displayed measurement result (optical signal).

Offset: The Offset value is subtracted from the measured signal to calculate the result. Offset can be set to zero or to the measured CW-value. Offset is useful to compensate for the influence of ambient light or if the measurement value is very small relative to the adjusted measurement range.

Integration Time: Time period for which the input signal is sampled and the average value of the sampled values is calculated (>CW). Integration time should be selected carefully. For example, if multiples of 20 ms (50 Hz) are selected as the integration interval, errors produced by the influence of a 50Hz AC power line can be minimized.

Sampling Rate: The rate which specifies how often the input signal is measured (sampled). The CW-value is calculated using the average value of all samples of one measurement interval (integration time). A sampling rate of 100ms means that 10000 samples per second are taken. If the measurement interval (integration time) is 0.5 s, there are 5000 samples used to get the CW value.

Slew-rate: how fast a signal changes. For example, a rate of 5 Volt/ms means that the signal changes with a value of 5 Volts every ms.

Rise-time: Time needed for a signal to change from 10% to 90% of its final value.

Fall-time: Time needed for a signal to change from 90% to 10% of its start value.

Input Ranges / Measurement Range: To achieve a dynamic measurement capability greater than six decades, different levels of measurement ranges (Gains) for the “current to voltage input amplifier” are necessary. Gains can span from 1V/10pA to 1V/1mA (depending on the device).

Linearity: The linearity of an optometer can be described as follows:

Reading a value of 10nA, with a max. gain error of 1%, the possible error is +/-0.1nA. Together with an additional offset error of 0.05nA, the total measurement uncertainty would be 10nA +/-0.15nA or 1.5%.

At a reading of only 1nA in the same gain range, the gain error would be 1% of 1nA or 0.01nA. The offset error would still be 0.05%. The total measurement uncertainty would be 1nA +/-0.06nA or 6%. The offset error is minimal with our optometers since these meters offer an internal offset compensation or allow an offset zero setting from the menu. Here the only offset error is from the display resolution or the nonlinearity of the analog-digital converter (ADC).

Measurement Accuracy / Linearity: The max. possible error of a measurement result can be calculated as follows:

Total Error: Gain Error + Offset Error

Gain Error: Displayed (or readout) result X (Gain Error (in percent) / 100 )

Offset Error: Constant value depending on measurement range
The Offset Error can be nearly eliminated be using offset compensation. Some errors cannot be compensated because they are produced by the nonlinearity of the ADC (Analog Digital Converter) and the display resolution.

Maximum Detector Capacitance: The input current-to-voltage ampflifier is sensitive to input capacitance. If the input capacitance is too large, the amplifier may oscillate. The maximum detector capacitance is the largest value of capacitance for which the amplifier will remain out of oscillation.

Measurement Range: The measurement range is typically specified by the resolution and the max. reading value. But the user should note that for a measurement with a max. measurement uncertainty of 1%, the min. measurement value should be a factor of 100X higher than the resolution. On the other hand, the max. value may be limited by the detector specifications such as max. irradiation density, max. operation temperature, detector saturation limits, etc., and therefore the manufacturer‘s recommended measurement values should be adhered to.

Fig. IV.1. Radiometer Schematic


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