@varunnayak, may I respectfully suggest that you don't use unnecessary, non-electronic, abbreviations, and that you start a new post for your request?
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Back to the topic...
Jahm, you really need to give proper specifications at the start, otherwise people come up with suggestions that are a waste of effort because they don't meet your requirements.
Most likely you can use an infrared LED and a photodiode, carefully aligned underneath your model. The photodiode will detect the reflection from the surface, and this will drop when a dark line is passed. That current drop can be detected and used as your distance pulse, if each line passed is all you need to measure.
If you need higher resolution, then Klystron's suggestion is what you need. Ultrasonic ranging is not expensive to implement, and it's probably easier to set up than trying to align the LED and photodiode.