Signal Processing

Detectors translate light energy into an electrical current. Light striking a silicon photodiode causes a charge to build up between the internal "P" and "N" layers. When an external circuit is connected to the cell, an electrical current is produced. This current is linear with respect to incident light over a 10 decade dynamic range.

International Light's Proprietary Amplification Technique

International Light radiometers amplify the induced current using a floating current-to-current amplifier (FCCA), which mirrors and boosts the input current directly while "floating" completely isoloated. The FCCA current amplifier covers an extremely large dynamic range without changing gain. This amplification technique is the key to our unique analog to digital conversion, which would be impossibe without linear current preamplification
We use continuous wave integration to integrate (or sum) the incoming amplified current as a charge, in a capacitor. When the charge in the capacitor reaches a threshold, a charge packet is released. This is analagous to releasing a drop from an eye dropper. Since each drop is an identical known volume, we can determine the total volume by counting the total number of drops. The microprocessor simply counts the number of charge packets that are released every 500 milliseconds. Since the clock speed of the computer is much faster than the packets, it can measure as many as 5 million, or as few as 1 charge packet, each 1/2 second. On the very low end, we use a rolling average to enhance the resolution by a factor of 4, averaging over a 2 second period. The instrument can cover 6 full decades without any physical gain change!
In order to boost the dynamic range even further, we use a single gain change of 1024 to overlap two 6 decade ranges by three decades, producing a 10 decade dynamic range. This "range hysteresis" ensures that the user remains in the middle of one of the working ranges without the need to change gain. In addition, the two ranges are locked together at a single point, providing a step free transition between ranges.
Even at a high signal level, the instrument is still sensitive to the smallest charge packet, for a resolution of 21 bits within each range! With the 10 bit gain change, we ovelap two 21 bit ranges to achieve a 32 bit Analog to Digital conversion, yielding valid current measurements from a resolution of 100 femtoamps (10^-13 A) to 2.0 milliamps (10^-3 A). The linearity of the instrument over its entire dynamic range is guaranteed, since it is dependent only on the microprocessor's ability to keep track of time and count, both of which is does very well.
What does all this mean to the user? Since International Light radiometers do not need to switch gain circuits to autorange, they can autorange during exposure integrations. In fact, International Light Radiometers can even autorange during flash integrations! There is no practical way to anticipate the intensity of a flash in the dark. Our radiometers can accommodate an incredible, unanticipated range change, instantaneously!
In contrast, a transimpedance (voltage) amplifier must change gain every decade, introducing errors to any dose measurement since it can't sample and change gain at the same time. Transimpedance amplifiers simply cannot autorange during a flash event without losing crucial data. This limits its dose and flash measurement capability to within a single range, since it cannot measure while changing gain. In addition, the linearity of the system is dependent on the gain adjustment between each decade.

Transimpedance Amplification

A graduated cylinder is a good analogy for transimpedance amplification. The major graduations indicate the top range, providing a good differentiation over a ten to one range, with the minor graduations indicating the resolution within that range. Incoming light induces a voltage, which is amplified and converted to digital using an analog to digital converter. A 10 bit A-D converter would provide a total of 1024 graduations between 0 and 1 volt, allowing you to measure between 100 and 1000 to an accuracy of three significant digits. To measure between 10 and 100, however, you must boost the gain by a factor of 10, because the resolution of the answer is only two digits. Similarly, to measure between 1 and 10 you must boost the gain by a factor of 100 to get three digit resolution again.
In the graduated cylinder analogy, changing range entails magnifying the view of the bottom 1/10 of the cylinder. To switch gain in the voltage amplification circuit, you switch to a different value feedback resistor to give 10 times the amplification. Now you can measure between 10.0 and 100. Similarly, you can switch to a gain of 100 and measure between 1.00 and 10.0. Each gain level must be adjusted independently in order to provide overall linearity.
In transimpedance systems, the 100% points for each range have to be adjusted and set to an absolute standard. It is expected for a mismatch to occur between the 10% point of one range and the 100% point of the range below it. Any nonlinearity or zero offset error is magnified at this 10% point. In a scanning or plotting system, where the reading changes gradually with wavelength, time, position, etc., the decade gain changes are obvious, appearing as an instantaneous step on an otherwise smooth curve.
The transimpedance amplification technique is used in nearly every measurement device. Volt meters, for instance, provide excellent accuracy from a milli-volt to over 10 volts (4 decades). While many meters measure either lower (micro-volts) or higher (kilo-volts), they rarely do both. Transimpedance amplification is excellent at measuring within a fixed range, but is inherently limited in its ability to cover a broad dynamic range. In applications such as a dual beam spectrophotometer, where the light source is a fixed part of the system, a 3 decade range is adequate.

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