# How to focus the flashlight of a smartphone?

How would I go about collimating the light from a smartphone's flashlight into a more focused beam? I'm working on a project where I need a focused beam of light to act as a light source so I can take pictures of the retina of a human patient using a smartphone. I'd rather use the built-in flash of the camera than build my own light source, but the light from the flash is very diffuse.

• There's a reason that optometrists' retinal illuminators are expensive: producing decent illumination after light passes thru the cornea and lens is not a trivial problem. I rather doubt you'll even have a lot of luck getting a smart phone to focus accurately on the retina. – Carl Witthoft Jan 23 '17 at 15:46

Focusing and collimating are two different things. Focusing means focusing into a small point (which will show the image of the light source); collimating is analogous to focusing into infinity, so that the beam diameter stays constant.

Either of these is readily done using a lens. The basic formula for lenses is:

$\frac{1}{S_1} + \frac{1}{S_2} = \frac{1}{f}$

Where $S_1$ would be the distance from light source to lens, $S_2$ would be either the focus distance or $\infty$ for collimating; and $f$ would be the lens focal length. You can choose two of these parameters quite freely, and then calculate the third one.

However, a word of caution: The flash light of a smartphone contains easily enough power to cause permanent damage to the eye. Especially if focused using a lens. Make sure you do the proper calculations and measurements for safety limits before trying this on anyone.

• It is also a bit tricky since the flash light and camera axis aren't collinear so getting the light to where the image is isn't guaranteed. The flash light focussing lens may easily get in the way of the imaging lens. If were me, I'd more thoroughly emphasize you word of caution. Perhaps bold red type, underlined. :) – Eric S Jan 23 '17 at 15:30
• [citation needed] I'm highly skeptical that the total output of the LED is sufficient to cause any kind of ocular damage. It's not a laser. – Carl Witthoft Jan 23 '17 at 15:43
• While what you state about Lensmaker's Formula is true, it's far short of what is needed to design a retinal illuminator. – Carl Witthoft Jan 23 '17 at 15:46
• Thank you all for your replies, I am a high school student working on this project on the side and I'm very new to the field. I have researched into the light intensity deemed safe by the FDA for the eye and the light from a smartphone is well within that limit. I will definitely emphasise the warning, through :) – kavyakvk Jan 24 '17 at 1:21
• @CarlWitthoft I'm not an expert on this, but isn't coherence mostly irrelevant for eye damage? So the main issue with lasers is that it is a tight beam from a point source, so that the eye will focus it to a very intense spot on the retina. It seems to me that a LED with a lens could do the same, and smartphone flash lights can have over a watt of power. – jpa Jan 24 '17 at 16:09