I live in a country hit hard by COVID-19, and stocks of sanitizers and disinfectants have run low. In order to save up on the limited stocks of sanitizer that we have, I thought of making a simple DIY germicidal chamber that can deal with microbes on non-living surfaces.

The idea is that anything inanimate can be disinfected with UV radiation, as long as the correct form is used. I have come across useful information from the Trojan UV website, which states the following important information about UV germicidal function

UV wavelength of between 200 and 300 nm is needed. The optimal frequency is 254 nm.
Germs are permanently deactivated when exposed to 12 mJ / square cm. Even a dose of 3 mJ / square cm is enough for some bacteria.

With this information, let's carry on with the build!

Supplies

Materials needed are

A large box. Any box that will fit what will be disinfected is good. It must have a cover or any other means of completely closing off the chamber.
Germicidal UV lights. Do not use UV lights used for black lamps and money scanners, as the wavelengths they emit are not correct, and hence would not disinfect well enough.
Aluminum foil to reflect UV light around the box. According to the fourth paragraph of the description in this patent, aluminum reflects UV radiation best.
A way to secure the foil into the box. In my case, I used aluminum tape.

Equipment needed are

Scissors
An electrical outlet

Step 1: Building the Chamber

The process is pretty straightforward. Simply follow these steps in order.

Crumple the foil, and stretch it back to a sheet. This provides semi-diffuse reflection, allowing the UV rays to permeate all over the box.
Cover the insides of the box with aluminum foil. This prevents UV rays from leaking out to the environment.
Install the germicidal UV lights inside the box.
Finish the electrical connections.

In my case, click on each image attached to go through the steps I followed to build mine.

Step 2: Calculations

And now, a few bits of science. TL;DR at the end.

The intensity of any type of electromagnetic radiation (light, radio waves, UV, etc) is defined as the amount of radiant power per unit area (I=P/A). Also, power can be defined as the rate of energy usage, i.e. P = E/t. Combining these two formulae gives the new equation I = E/At.

We know that microorganisms are rendered dead when exposed to 12 mJ / square cm. Since this is already in the form of E/A, we can now get a new equation, I = (12 mJ / square cm) / t. Rearranging everything gives us

t=(12mJ / square cm) / I or

t = (0.012 J /square cm) / I

This is the formula we will use to calculate how long to turn the UV chamber on.

The lamps I got were 8W lamps. According to this data found in ResearchGate, the minimum intensity of UV each lamp could put out is 520 uW / square cm, or 0.00052 W/square cm. Since I am using 2 UV lamps and placing them side by side, the total intensity achieved of approximately 0.001 W/ square cm. In your individual cases, check out the intensity of UV radiation in the table and plug it into the equation above.

Plugging in this value into the formula gives us a time of

t = (0.012 J /square cm) / (2 x 520 uW/ square cm)

t = (12 mJ /square cm) / (1040 uW/ square cm)

t = (12000 uJ /square cm) / (1040 uW/ square cm)

t = 12 s (approx.)

This is the minimum time needed to expose the objects to UV radiation in the chamber in order to be disinfected. To be on the safe side, objects will be exposed to UV radiation for 15 seconds; afterwards they are placed in a new position and exposed for another 15 seconds.

TL;DR - to calculate the minimum time (in seconds) needed to expose the objects in UV rays, divide 12,000 by the smallest power value (in microwatts {uW}) for the lamp you used, then divide again by the number of lamps you are using.

Now, let's see the chamber in action.

Step 3: Safety With UV, Usage Instructions, and Live Tests

First of all, the container must be sealed in a way that does not leak any UV radiation when being used, as this is of the type that can cause severe sunburns in the short-term, and skin cancer in the long run. You can see in the videos that the box keeps the UV rays inside.

Watch the videos of the box in action!

Step 4: How to Use This Thing?

POP, TURN ON

if the chamber is used as-is, with no modifications to how items are placed (i.e. a platform), then the best way to disinfect items is to

with the lights off, place the items inside the chamber
place the lid.
turn on the UV lamps for 15 seconds.
turn off the lamps, and open the lid.
flip the items over
repeat steps 2 to 4 with the flipped objects.
take out the items, ,and done!

NO NEED TO UNWRAP

The same steps can be used with any items covered with wrapping that is transparent to UV light. This includes

thin-film plastics (shopping bags, cling wrap, a few layers of bubble wrap)
transparent materials, except glass
thin cardboard

UNWRAP

If the items are covered in anything else, they must be removed from the wrapping. Wrapping materials that block UV include

glass (depends on type and thickness, but safe to assume that glass will always block UV)
thick cardboard (think packaging boxes)
white paper (treated with a UV-absorbing material that makes the paper glow white)
thick plastics (the thicker the plastic, the less effective disinfection of the items inside are)

Also, unwrap anything that are relatively large and usually piles up into a bunch, like fruits and toys.

I hope you enjoyed this Instructable! Questions are always welcome!