laserpointer circuit
Sure, that is simplest way to use the direct measurement of diffusion reflection light. And modulation of source light need for this.
In lab. experiments I used the laser pointer, directly modulated from uC. Life time of the laser pointer decreese, but not signifacant for me.
May be more good solution for you is to use power infrared LED.
There are two reasons for this - 1. you don't need reflection from the narrow area 2. max of sensitivity the Si-photodiodes in IR (~1000nm) , that corresponding with the emmitted IR LED's light (~800-950nm). In other words, to use the red laser pointer with ~650nm (were the sensitivity of the Si-photodiodes aproxim. 1/2 max), not effective.
To avoid influence from ambient light, you need good photo-recieve-system with big dinamic range. This is possible, if used ph-diodes or optoreceivers like OPT101, OPT201. To avoid saturation of optoreceiver fix small gain for direct current ( constant light ) in stage convertion photocurrent-voltage. For example, set external resistor for OPT101 less then 50k ( although native 1M for this chip), use capasitor an output. You need post-amplificaition with high gain only for AC component in narrow band of you's modulation frequency.
Very best for detection signal is to use a synchronous detection.
For additional, it is very good idea to use optical filtr before optorecievers.
In case IR light system you can use the filtr from TV remote control
.
It's usefull for you, that fluorescent lighting give very big disturbance. It have 100% modulation of light, in comparison with 20-30% incandescent lamp. One more reason to use IR light.
good luck!
PS: sorry for my English.