One of the reasons our photometry modules have given such good results over the years is that they normally work by integrating the optical signals over an appropriate period of time. This form of operation is particularly useful in dual or multiple excitation systems, since the integration period can be matched to the time during which a particular excitation wavelength is present, and the integrated output is a direct measure of the total amount of light detected during the integration period. For continuous measurements, a much simpler system is often used, in which the detected optical signal (usually supplied as a current) is converted to an output voltage, which is then filtered to some degree to reduce noise. However, the problem with this type of arrangement is that the filtering needs to be matched to the sampling rate of the subsequent data capture system. Insufficient filtering for a given
sampling rate means that the data will be more affected by high-frequency noise, whereas too much filtering means that the response time will be increased, so that genuine signal changes will be smeared out over a number of successive data samples.

By using an integration method, and matching the integration time to the sampling rate, this problem is removed. For a given light level, the output depends on the integration time, but in practicethis is a very useful additional way of controlling the gain of the system. However, additional electronics are necessary to generate the timing and control signals for operating the integrator, reducing the convenience of the integration method. To solve this problem we introduced the Cairn Integra as a single-box solution incorporating all the circuitry of our standard input amplifier module, together with an integration controller. Thus the Integra can function either as a standard currentvoltage converter system with variable filtering, or as an integrator with variable sampling time. The sampling time of the integrator can be controlled either by an internal timer, or by an external control signal, so it is very easy to use. We've included a photomultiplier power supply, but it can also be used with photodiodes, so we think it should be a pretty universal solution.