Have you looked up such chip manufacturer that is called Burr-Brown (now part of TI). They have 4..20mA chips both receivers (RCV420) and transmitters (XTR115, XTR116).
You might get better answers if you give the information what is now in your light sensor design.. Ie. which format is the output in (Hz, Ohm, Current, Voltage, PWM, etc.)
Sensor -> Amplifier -> Interface
One option is to use voltage output and convert the output to 20mA max output with some resistors and Ohms law. This would be the quick&dirty option.