The present invention provides core technologies necessary for a portable, low cost LED-based handheld fluorometer. The fluorometer is based on a heater integrated within the walls of the reaction chamber, and an orthogonal geometry LED based light source to provide optical excitation. Power is supplied through either an internal power supply, and data is collected in real-time through standard serial interfaces of personal computers, which can also be used to provide power, or personal digital assistants. Thermal regulation is automatically maintained using temperature sensor feedback control. Such a handheld system can allow applications requiring temperature sensitive photometric measurements for real time analyte detection to be performed in the field.