Cheap lenses are getting better! It is now possible to make an inexpensive digital microscope using a phone with a camera coupled with another lens. There has been a huge amount of interest in doing this as demonstrated in many Instructables. While these do not replace expensive digital microscopes with high magnification, they are quite useful, especially in the 50x to 150x range. This instructable shows how to do it on a large scale to outfit your friends and relatives on a very inexpensive per lens basis.

First, though, here is a review of some of the methods featured in Instructables so far: Here are links to several that use laser pointer lenses:

https://www.instructables.com/id/10-Smartphone-to-digital-microscope-conversion/

https://www.instructables.com/id/How-To-Make-Simple-Smartphone-Microscope-40x-170x-/

https://www.instructables.com/id/Turn-Your-Smartphone-Into-a-Microscope-150x-500x-Z/

https://www.instructables.com/id/Create-a-mobile-microscope/

This one features a disposable camera lens:

https://www.instructables.com/id/Macro-lens-for-phone-camera-iPhone/

This one uses a binocular lens:

https://www.instructables.com/id/how-to-make-a-microscope-lens-for-a-phone/

This one uses a webcam lens:

https://www.instructables.com/id/Smartphone-DIY-Microscope/

This one uses an LED lens:

https://www.instructables.com/id/Phone-Marco-Photography-HQ-using-LED-lens/

An elegant approach featured in these three Instructables uses a hanging drop of water on the phone camera’s lens to provide the supplemental magnification:

https://www.instructables.com/id/How-to-make-a-water-drop-macro-lens/

https://www.instructables.com/id/Any-cheap-camera-phone-water-drop-Macro-photograph/

https://www.instructables.com/id/Homemade-macro-lens-for-camera-phones/

This works because the drop approximates a circular paraboloid, which is a pretty good shape for a lens. There is another way to do this, with a hanging drop of clear silicone cured to make transparent rubber. This has been published:

W. M. Lee, A. Upadhya, P. J. Reece, Tri Giang Phan. Fabricating low cost and high performance elastomer lenses using hanging droplets. Biomedical Optics Express, 2014; 5 (5): 1626 DOI: 10.1364/BOE.5.001626

To my knowledge, however, this method has not yet appeared as an Instructable. The idea is to build it up in layers. The first drop is not hanging, just a simple drop of the 2 part silicone on a glass surface, such as a slide. After it is cured, a second drop is placed on top of it, and the slide is inverted so the drop cures as a hanging drop. A third drop can be added in the same way, increasing the magnification of the lens produced while decreasing its focal length and decreasing the area covered. This Instructable explains how to do it.

Step 1: Assemble Materials

Materials needed:

Sylgard 184 - This is a two component clear silicone rubber typically used to encapsulate electrical components to protect them from moisture, dust, vibration, etc. It is quite expensive. I paid about $80 with shipping for a one pound kit. But this kit is probably adequate to make more than a thousand lenses.

Oven – This silicone cures quite quickly at fairly low temperatures. I used 250 F for 30 minutes.

Microscope slides – any clean glass surface works, but slides are inexpensive and convenient.

Eyedroppers – At least two! It is very important not to contaminate on component of the silicone with the other component. Keep track of which eyedropper is used for each! Don’t use them for anything else. Syringes can be used instead of eyedroppers if you like.

Support for the slides with hanging drops - I taped cardboard strips to the bottom of a Pyrex baking dish .

Single edge razor blades to remove the finished lenses from the slides.

A container to mix the silicone – I used plastic milk bottle tops. These were more than adequate for my very small batches.

A stick to mix the silicone and distribute the drops to the slides – I used a small wooden matchstick.

Step 2: Preheat the Oven, Mix the Silicone

Preheat the oven. I used 250 F.
There are a variety of heat treatments that work. See the Sylgard 184 specs:

http://www.dowcorning.com/applications/search/products/Details.aspx?prod=01064291

Mix the Sylgard 184 components well in the 10:1 ration specified by the manufacturer. Make sure there is no possibility of one part contaminating the other in their original containers. A very small batch is best. I mixed mine in a plastic milk bottle cap.

Step 3: Prepare Drops

Put about 4 0r 5 drops on each slide, giving them enough space so they don’t contact each other when they spread out. I used a small wooden stick match (wood end) to mix and distribute the silicone. I let the drop fall from the matchstick onto the slide. That is certainly not the only way to do it, but it was easy and made drops of similar sizes.

Step 4: Bake First Drops

Put the slides in the preheated oven on a cookie sheet for about 30 minutes if using the 250 F setting, or else by other Sylgard 184 specs.:

http://www.dowcorning.com/applications/search/products/Details.aspx?prod=01064291

Remove from the oven after the specified time and let cool.

Step 5: Prepare Setup for Baking Additional Drops and Use It

While your first drops are baking, prepare a setup such as a couple strips of corrugated cardboard taped to the bottom of an inverted Pyrex baking dish to allow flipping the slides over to create hanging drops. The strips should be thick enough to give the drops room to hang. After your first drops have cooled, set the slides right side up using the strips to support their ends. Place a second drop on top of each of the first cured drops on a slide. Let these drops spread out to nearly cover the first drop. Then invert the slide so the new drops are hanging drops. Repeat with each slide. Bake as in Step 3, and remove to cool.

Step 6: Optional Additional Drops

Repeat Step 5 if a third drop is desired. Repeat again if a fourth drop is desired. I have used two drop, three drop, and four drop lenses so far. I like the two drop lenses best. Adding drops increases the curvature of the lens, giving it greater magnifying power, a shorter focal length, a smaller depth of field, and a smaller field of view. It also makes the lens trickier to line up with the phone camera lens, and trickier to focus. Working time of the mixed silicone is greater than 90 minutes, so it is possible to use one batch of mixed silicone for several layers, or to make another batch of single drops, etc.

Step 7: Harvest and Use Your Lenses

Remove the cooled lenses from the slides with new single edge razor blade. Place one over the camera lens of your phone. Press it on. It will stay there well enough to use, but is easy to remove. If you forget it is there, eventually it will fall off and you may lose it. A dime or other coin makes a pretty easy subject to explore first. You can easily get a feel for the right focal length, what lighting works best, etc. Remember your phone is still a camera. Take some pictures or videos!

Step 8: An Optional Stand

I very much like the stand in this
Instructable:

https://www.instructables.com/id/10-Smartphone-to-digital-microscope-conversion/

Once you have adjusted it so your subject is in focus, your hands are free to take pictures, move the subject around horizontally, etc. Of course if you are using a silicone lens there is no need to build the laser pointer lens featured in the instructable into the stand.