5 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 ranging from 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 range. Selectable averaging calculation of the sampled results from microseconds to seconds provides more measurement flexibility for fast events or lowlevel signals. The device can be operated via a logical menu structure where user input is done through a front-panel keyboard or computer control via RS232 or IEEE computer interface.
The quantity or optical unit measured will depend on the detector type, its configuration in terms of filtering and input optic, and its calibration. Radiometers are available in hand-held mobile and bench-top models for laboratory use. Selfcontained 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, data logging) differentiate the various models. The type of application usually determines the specific capabilities that a radiometer system should have. For example, in a UV curing production process where multiple stations must be monitored, a multi-channel radiometer with an adjustable minimum / maximum reading feature, RS232 or IEEE interface and remote multiplexed detectors would be desirable.
Fig. 1: Radiometer schematic
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 and Features
| CW | Continuous wave is a run of continuous type measurements. The measurement frequency depends on the integrating time and the maximum sampling rate of the meter |
| CW Min / Max | CW measurement where the minimum or maximum value that occurred during the measurement run will be displayed. The min. or max. value can be reset via the RESET switch. |
| CW Level Check | CW measurement where the measurement values are compared with min.- max. threshold values. The threshold values are entered into the meter by the user. |
| CW Level Minimum / Maximum | Menu to adjust the threshold values for CW Level Check. |
| Run / Hold | Freezes a measurement value on the display and stops 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 individually saved in the memory of the optometer. Each measurement may be triggered manually or automatically through a preset measurement cycle time. Measurement data can be output via a 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 highest positive sample of a 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 least negative sample of a 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 between the highest and least sample of a 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 pulseenergy is calculated by integrating the samples. IEffective 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 0.1 s and 0.2 s, 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. The basic configuration uses 3 pins (of a DB-9 type connector): ground, transmit data and receive data. On PCs, the RS-232 ports are either marked as “serial” or “asynch” and are either of 9 or 25 pin male type. |
| Remote IEEE488 | The device’s IEEE488 interface is enabled. IEEE488 is a standard for parallel transfer between computer equipment and measurement instruments. Data is transmitted in a 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 can be connected to one USB host computer. The connected devices can be powered by the host computer. |
| Ethernet | Is a technology that specifies software (protocols, etc.) and hardware (cables, distribution stations, network interface cards, etc.) for tethered data networks. It was originally intended for local area networks and hence the term LAN technology. It enables data exchange via data frames between locally tethered devices (computers, printers and the like) within the network. Currently, transfer rate of 10 Megabit/s, 100 Megabit/s (fast Ethernet), 1000 Megabit/s (Gigabit Ethernet), as well as 10 Gigabit/s, 40 Gigabit/s and 100 Gigabit/s are specified. |
| Auto Range | When activated, the measurement range is automatically switched by the device to the optimal value (depending on the input signal). |
| Manual Range | When auto range is disabled, the measurement range can be manually fixed to a certain value. The device is not allowed to automatically switch measurement ranges. Manual range adjustment can be useful in cases where input signals change 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. The offset can be set to zero or to the measured CW-value. This function is useful in compensating for the influence of ambient light or if the measurement value is very small with regards to the adjusted measurement range. |
| Integration Time | Time period for which the input signal is sampled and the average value of the sampled values 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 50 Hz 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 100 ms 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. |
Tab. 1: Operating Modes and Features
Specifications
| Slew rate | Shows 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 millisecond. |
| 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 | In order 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 1 V / 10 pA to 1 V / 1 mA (depending on the device). |
| Linearity | The linearity of an optometer can be described as follows: Reading a value of 10 nA, with a max. gain error of 1 %, the possible error is ± 0.1 nA. Together with an additional offset error of 0.05 nA, the total measurement uncertainty would be 10 nA ± 0.15 nA or 1.5 %. At a reading of only 1nA in the same gain range, the gain error would be 1 % of 1 nA or 0.01 nA. The offset error would still be 0.05 %. The total measurement uncertainty would be 1 nA ± 0.06 nA 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 eliminated through offset compensation. Some errors cannot be compensated for 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 amplifier 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. The user should however 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. |
Tab. 2: Specifications