# How can I predict the spread/span of a single IR beam?

I want to project an IR field across a 5 inch hole to detect if an object passes thru it like a mouse, a ball or other such object; say a 4 inch ball, how can I determine or predict the spread or span from the point of origin of a single IR beam across the 5 inch hole?

Ultimately I want to know when the IR field is broken and register the event.

I have NO real practical experience with electrical engineering and realize I may just have to find an engineer in the real world and discuss this but thanks.

• 4" ball will surely break a beam crossing the 5" hole along it's diameter. – Eugene Sh. Mar 9 '18 at 21:22
• Spread of the beam is less of an issue for a typical LED. Your issue is having a receiver in the right place to go into the shadow of the passing thing. – Trevor_G Mar 9 '18 at 21:24
• The mouse is tiny compared to a 5 inch hole so it may not interrupt the beam at all. Maybe a motion sensor would be more appropriate? Can you make a drawing of the contraption? – peufeu Mar 9 '18 at 21:29
• Every mouse I've seen has feet, feet that are touching the ground. So if you would place the IR sensor a couple of cm above the floor and pointing in parallel with the floor, then you will most likely detect anything that passes through the hole. - This is assuming the hole looks like in Tom and Jerry. If your hole looks like something else, like pointing upwards or downwards or whatever, then it's a real shame that you thought it was not worth sharing that information. – Harry Svensson Mar 9 '18 at 21:37
• If you use a differential approach it might be pretty trivial if there is limited external light hitting the scene. If you have to deal with room light at 100/120Hz, sunlight, moving shadows etc. it might be non-trivial. – Spehro Pefhany Mar 9 '18 at 21:41

You have the 'spread' of the beam coming out of the IR diode. You will find that from the angle diagram of the IR diode.

Here is a diagram of an IR diode specified as 120 degrees:

A receiver also has a 'viewing angle'. You have to see if the shadow of the object is well withing the viewing angle. That is more physics (although very simple physics).

Simplest way to improve coverage area is to use more then one detector. Because they conduct when light falls on them you can put them in series.

Very important is to make sure no other IR objects are radiating into your sensor. (e.g. the sun)

Post edit:
Of course you can do what they show in the movies: have a series of mirrors bouncing the beam back and forth from top to bottom.

(Make sure to leave a big gap at the bottom for the hero to crawl through.)

• Thanks everyone. Sorry I did not mention that it is possible the hole might be vertical or horizontal both do exist and they are just round holes. What everyone here has shared with me has given me the insights that I lacked and I have learned something. I never thought about the amount of direct light; sunlight or bright room light, which is something I need to now consider although I think it will have limited impact on the IR beams. I will add a few more details after I consider a few things you all have made me think about now. Thanks again for your patience. – Tom Person Mar 9 '18 at 22:04
• the standard trick to deal with ambient light is to modulate the transmitted beam, and only detect at that frequency. – Neil_UK Mar 9 '18 at 22:28