One or more option exists to achieve your requirements, lets divide your project into 2 segmentations
Segment 1 "Choosing the suitable sensor for your circuit"
LDR (Light dependant resistor) as its name implies, this type of resistors are similar to regular resistors but resistance change is dependent on the amount of light it’s exposed to (high light intensity -> low resistance -> high output voltage) and vice versa. So by applying a 5v supply to one end of the LDR you can get an output voltage dependant of that amount of light at the other end of the LDR as shown on this circuitry " figure 1"
The most confusing part of this circuit is that LDRs output voltages are analog means you have to either connect them to analog pins of your micro-controller which are few compared to digital ones, or you will have to add some circuitry to convert those analog voltages into 5v digital voltages adequate to your micro-controller. to do that you will have to first test how much voltages are at the output of the LDR pin when the SMD LED is turned OFF and after exposing it to your SMD LEDs. you will be lucky if you can have a value higher than or equal to 2v when the SMD is ON as some micro-controllers such as the AT-Mega32 accepts high digital input values between 1.8v to 5.5v. otherwise you are going to determine the ON and OFF voltages of the LDR output and add this circuit "figure 2"
Where IC1 is a 5v comparator op amp, Vref is a voltage between the ON and OFF voltages of the LDR so that when SMD LED in ON, LDR output voltage is higher than Vref and comparator output is high (5v) to the microcontroller pin. And when SMD LED is OFF LDR output voltage is less than Vref and comparator output is low (0v).
Photodiode Photodiodes are another type of light sensor. But instead of using the change in resistance like LDR, it’s more complex to light, easily changing light into a flow of electric currents. To convert that current into voltage, a simple circuit with an operational amplifier allows you to get an output voltage proportional to current generated by the photodiode as shown in "figure 3"
The value of R can be calculated from this equation Vo = I * R , where Vo is the output voltage to the micro-controller pin (5v in our case) and I is the amount of current produced when the photodiode is exposed to SMD LED ON. R can also be determined by a try and error algorithm. assume it 50K Ohms and measure the output voltage then increase or decrease it according to requirements.
Segment 2 (Attaching your diagnostic PCB to the SMD LED PCB)
You can measure the exact board dimensions then measure the location of each SMD LED on that board reference to board dimension , then placing whichever sensor you have chosen into the same exact location of each SMD LED on the board and finally take in consideration that you are going to flip that new PCB to be placed over the existing one. Then using some kind of a 3d printed tubing around each LED and Sensor in order to eliminate the interference between lights from different SMD LEDs will be adequate.
