Background

In this report you can find information about blacklights, components and uses for it.

I chose this topic because I was interested about how other wavelengths of the light spectrum could affect to how human see objects and colors. I’ve discovered that ultraviolet light can produce visible light in objects because of the phosphors components and the energy that is transmitted to them producing a visible light for the human eyes.

This report is divided in introduction, questions, definition of terms, answers, my conclusion and some references I used.

Introduction

Our eyes can see visible light in a spectrum ranging from red through orange, yellow, green, blue and violet. In figure 1 you can see that above violet is ultraviolet light, which we cannot see but black light bulb can produce UVA light (as opposed to UVB light, which is much more harmful). [1]

Figure 1. Ultraviolet light frequencies are above violet on the spectrum and are invisible to the human eye

Questions

So, after the introduction some questions came into my mind:

• What are the differences between UVA and UVB?

• What you see glowing under a blacklight?

• What are the components of a blacklight bulb?

• How can you see substances emitting visible light in response of a non visible light source?

• For what we would like to have these kind of light sources?

Definition of terms

• UVA Rays - Stand for Ultraviolet A or more easily remembered as "UV Aging rays"- they are the cause of long term skin damage & photo-aging. In other words, they cause premature ageing, wrinkles and sun spots.

• UVB Rays - Stand for Ultraviolet B and are often referred to as "UV Burning rays"- they are the cause of sunburn. Unlike UVA, they have different strengths year round. UVB rays are the common cause of most skin cancers.

• UVC Rays - Stand for Ultraviolet C. It is the strongest and most deadly of solar rays, however the ozone layer stops these from reaching the Earth.

Figure 2. Ultraviolet light division

• Phosphors: A substance that can emit light after absorbing some form of radiation. The insides of television screens and fluorescent lamp tubes, for example, are coated with phosphors.

• Fluorescence:  The emission of electromagnetic radiation, especially of visible light, stimulated in a substance by the absorption of incident radiation and persisting only as long as the stimulating radiation is continued.

• Light: Electromagnetic radiation of any frequency or wavelength.

• Photon: The smallest unit of light or other electromagnetic energy, having no mass and no electric charge. Photons behave both as particles and waves.

• Inert gas: A gas that is not chemically reactive, especially a noble gas.

Answers

What you see glowing under a blacklight, whether on a fluorescent poster or an invisible hand stamp or a newly washed white T-shirt, are phosphors.

A phosphor is any substance that emits visible light in response to some sort of radiation, converting the energy in the UV radiation from a black light into visible light.

White T-shirts and socks normally glow under a black light because modern detergents contain phosphors that convert UV light into white light. This makes whites look "whiter than white" in normal sunlight. What you are seeing in sunlight is the normal reflection of visible white light from the cloth, as well as the emission of white light that the phosphors create from UV light in sunlight. The T-shirt really is whiter than white!

Black Light Designs

The conventional black light design is a fluorescent lamp with a couple of important modifications. Fluorescent lamps generate light by passing electricity through a tube filled with inert gas and a small amount of mercury. When energized, mercury atoms emit energy in the form of light photons. They emit some visible light photons, but mostly they emit photons in the ultraviolet (UV) wavelength range. Because UV light waves are invisible to the human eye, fluorescent lamps have to convert this energy into visible light. They do this with a phosphor coating around the outside of the tube.

Phosphors are substances that give off light (or fluoresce) when they are exposed to light. When a photon hits a phosphor atom, one of the phosphor's electrons jumps to a higher energy level, causing the atom to vibrate and create heat. When the electron falls back to its normal level, it releases energy in the form of another photon. This photon has less energy than the original photon, because some energy was lost as heat. In a fluorescent lamp, the emitted light is in the visible spectrum so the phosphor gives off white light we can see. Black lights work on this same principle, like in the figure 3.

Figure 3. How atoms emit light

There are actually two different types of black light, but they work in basically the same way:

• A tube black light is a basically a fluorescent lamp with a different phosphor coating. This coating absorbs harmful shortwave UV-B and UV-C light and emits UV-A light (in the same basic way the phosphor in a fluorescent lamp absorbs UV light and emits visible light). The "black" glass tube itself blocks most visible light, so in the end only benign long-wave UV-A light, along with some blue and violet visible light, passes through. In figure 4 you can find an example.

• An incandescent black light bulb is similar to a normal household light bulb, but it uses light filters to absorb the light from the heated filament. It absorbs everything except the infrared and UV-A light (and a little bit of visible light). In figure 4 you can find an example.

Figure 4. Black lights come in both tube and bulb form

In both of these light designs, the emitted UV light reacts with various external phosphors in exactly the same way as the UV light inside a fluorescent lamp reacts with the phosphor coating. The external phosphors glow as long as the UV light is shining on them.

Figure 5. Some ink reacting to a blacklight

Black Light Uses

If you walked around all night with a portable black light, you would discover that there are phosphors all over the place. There are lots of natural phosphors, in your teeth and fingernails, among other things. There also a lot of phosphors in man made material, including television screens and some paints, fabric and plastics. Most fluorescent colored things, such as highlighters, contain phosphors, and you'll find them in all glow-in-the-dark products.

Figure 6. Some articles that use blacklight

Clubs and amusement parks use special black light paint that glows different colours. You can also buy fluorescent black light bubbles, invisible black light ink, fluorescent black light carpet and even fluorescent black light hair gel.

In addition to making people and fluorescent posters look cool, black lights have some practical applications. For example:

• Appraisers use them to detect forgeries of antiques. Many paints today contain phosphors that will glow under a black light, while most older paints do not contain phosphors.
  

• Repairmen use them to find invisible leaks in machinery -- they inject a little fluorescent dye into the fuel supply and illuminate it with a black light. For example, they might detect an invisible air conditioner leak by adding fluorescent dye to the refrigerant. Black lights can be used to detect counterfeit bills.

• Law enforcement officers can use them to identify counterfeit money. Many countries include an invisible fluorescent strip in their larger bills that only shows up under a blacklight.

Figure 7. UV light for law enforcement officers

• Amusement parks and clubs use them to identify invisible fluorescent hand stamps for readmission.

• Forensic scientists use them to analyze crime scenes. To pick out fingerprints, for example, they often dust with fluorescent dye under a blacklight. This makes it easier to pick the fingerprints out from surrounding dirt. Black lights can also identify bodily fluids that naturally fluoresce.

Figure 8. UV light in criminal investigations

Most of these uses, as well as dozens of others, follow a common theme, the black lights make the invisible visible or isolate one specific substance from everything around it. When you think about it, there are dozens of situations where you could put this phenomenon to work. The applications are potentially endless!

Conclusion

For concluding, is possible to generate visible light using a light out of the wavelength of visible light with our own eyes. There are many things we don’t see that they are there. Just think the small wavelength human can see and big wavelength light has.

I don’t know, maybe in the near future we can have sun glasses where you can change the wavelength of light you want to see.

References

1. “Banana Boat Has a Great New Range of SPF50  Sunscreen on the Shelves Now!” What are UV Rays? UVA, UVB &amp; UVC. N.p., n.d. Web. 15 May 2015. <http://www.bananaboat.com.au/sun-facts/what-are-uv-rays/>

2. Harris, Tom. “HowStuffWorks.” HowStuffWorks. HowStuffWorks.com, n.d. Web. 15 May 2015. <http://science.howstuffworks.com/innovation/everyday-innovations/black-light.htm>

3. “How Light Bulbs Work - HowStuffWorks.” HowStuffWorks. N.p., n.d. Web. 15 May 2015. <http://home.howstuffworks.com/light-bulb.htm>

4. “Re: What Gases Are in a Black Light Bulb and How Does It Work?” Re: What gases are in a black light bulb and how does it work? N.p., n.d. Web. 15 May 2015. <http://www.madsci.org/posts/archives/mar97/854032426.ch.r.html>

5. “USATODAY.Com - Black Light Makes Detergent Phosphors Glow.” USATODAY.com - Black light makes detergent phosphors glow. N.p., 2001. Web. 15 May 2015. <http://usatoday30.usatoday.com/news/science/wonderquest/2001-06-27-black-light.htm>