Fundamentally, good lighting design starts with the source more so than the fixture. I’m going to kick things off by breaking down what you need to know about how artificial light is created, how our eyes interpret light and color, how it’s measured, and we’ll wrap up this article with information what it all means, to help guide you to make good bulb choices in your next purchase.
The sun, our natural light source, operates on nuclear reactions, producing photons that travel across the vastness of space to reach us. Artificial light sources, on the other hand, like bulbs and screens, generate photons through electrical energy and chemical reactions. Each method of light creation has a different impact on the type and quality of light produced.
Examining light is not just about what’s visible; it’s also about understanding its place in the electromagnetic spectrum, as shown below. Visible light falls within a range of wavelengths, sandwiched between ultraviolet (UV) light and infrared (IR) light. This section of the spectrum is pivotal because it’s the only part our eyes can see, which influences how we interact with our environments.
You can also see within the visible spectrum the range of different colors. This is important when we talk about color rendering and color temperatures and how those measurements affect what we see.
How is Artificial Light Created?
Electric lighting comes from the conversion of electric energy into light. The energy source radiates, or gives off, energy. We recognize these sources generally as bulbs, but also as tubes such as fluorescent or LED tubes.
Traditional electrical light is created through Incandescence. This is the process of burning, much like fire. When you turn on a light, it ignites the filament in that bulb. And much like fire, the traditional household incandescent bulb used 90% of its energy to create heat, which is why they are far less energy efficient than a fluorescent or LED bulb. However, that heat output does actually supplement your home heating in the winter, and why it feels cooler in the summer if we turn the lights off. But I definitely don’t recommend trying to heat your home with these bulbs!
Arc Discharge is the process of discharging an electric current (or arc) through a gas, Lightning is the natural form of this phenomenon, and also how fluorescent and halogen lights work.
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons.
The Human Connection: How Our Eyes Interpret Light
Let’s explore how our eyes act as the ultimate light receivers, turning photons into images. The human eye is a marvel of nature, complete with its specialized cells known as rods and cones. Rods are the workhorses in low-light conditions, allowing us to detect shapes and movements even when it’s pretty dark out. Cones, on the other hand, give us the power to see a spectacular array of colors when the lights are up.
Now let’s break down the journey of light from the outside world to our brains. It starts with photons zipping into our eyes, hitting the retina at the back. They trigger those rods and cones, which then fire off signals to the brain through the optic nerve. Your brain processes all of these signals to create the images you see before you. But, we cannot determine the absolute quantities of light directly. Our eyes and our brain do not have a “meter” to measure that quantity. Rather, we perceive the contrast in brightness within and between objects. What we see is brighter than or darker than other objects.
Light is a key player in how we perceive and interact with the world around us. This brings us to the question – how do we measure this pivotal force if we don’t have a meter in our brains? That’s going to include an understanding of lumens, footcandles and candela, which we cover in a further section.
A Spectrum of Possibilities: Color and Perception
Have you ever stopped to think why the sky is blue or why the grass looks green? And all of the variations of each? It has much to do with the color of light. Light reflects off of an object sending the signal to the brain. The colors in those signals are dependent on the colors present in that light. Different light sources have different mixes of colors dependent on the wavelengths that make up that light, and the light can’t reflect colors that aren’t present within that light. This is why the colors in your home seem to change between light sources, and the color of a paint chip looks different outside under natural light, from inside the store.
Let’s dive into the two big terms that aren’t just scientific jargon but part of our everyday decision-making when we buy light bulbs: color temperature and color rendering.
Color temperature is measured in Kelvin and it helps us understand whether a light source appears warm, like a candlelight, or cool, like a cloudy day. If you look back to the image at the top of this page, you see the colors of the rainbow with red on the far left and gradually becoming cooler (more blue tones) as you move to the right. When you think of Kelvin, the measurement of color temperature, remember that lower numbers are warmer and moving left to right, like reading this article, Kelvin number gets higher for cooler light. An incandescent bulb, or sunlight shortly before sunset, is around 2700 Kelvin; a halogen bulb around 3000 Kelvin, and it’s generally accepted that high-noon daylight in the summer on a clear day is around 5000 Kelvin.
Color rendering, on the other hand, is about how well a light source can reveal the colors of various objects faithfully. This is where the Color Rendering Index (CRI) comes into play. A bulb with a high CRI is like that friend who always tells you the truth — it shows colors accurately without adding any twist. Look for a bulb that has a CRI of at least 90, the higher the better. Lower CRIs, such as 80 commonly found in fluorescent bulbs and tubes, and can leave colors feeling flat.
The quality of light, the Kelvin and the CRI, can significantly affect the ambiance and functionality of a space.
Quantifying Illumination: The Metrics of Light
When you’re trying to grasp the intensity and quality of light, there are a few key metrics that experts rely on. The most common units of measurement are lumens, footcandles, and candela. As consumers we’ve understood a traditional incandescent bulb’s light output in terms of watts. A watt measurement is the amount of electrical energy the bulb consumes. If a bulb is 100w, we have a perception of the amount of light that bulb puts out when we get it home because of our experience with those bulbs. But as fluorescent, halogen and now LED technologies are more prevalent in the market, we can no longer chose bulbs based on that perception and a shift to expressing light levels through lumens has begun.
Lumens measure the total amount of visible light emitted by a source, which basically tells you how bright a light bulb is. Imagine it as the ‘output’ of the light bulb or, in simpler terms, the amount of light you’re getting. A common 60w incandescent bulb emits approximately 800 lumens, whereas we can get the same lumen output from a 45w halogen, or a 9 watt LED.
Lumens per watt is an expression of the efficiency of a bulb. In the above example, the incandescent bulb produces just about 13.3 lumens per watt. The 9 watt LED is producing around 88.9 lumens per watt. Big difference, right? You get much more light for your money with LED, because the incandescent uses so much of it’s energy consumed to create heat by burning the filament to create light. This change in terms and measurement has created significant confusion for consumers but recently manufacturers have updated their packaging to demystify the new lingo.
Footcandles on the other hand, measure the amount of light that hits a surface. 1 lumen falling on one square foot is called a footcandle. In metric terms, one lumen per square meter is called 1 lux. It considers both the light’s intensity and the area over which it spreads. To put it plainly, while lumens tell you how much light a bulb produces, footcandles or lux, tells you how much of that light actually reaches a designated spot you might want to illuminate.
Then there’s candlepower that describes the luminous intensity in a specific direction. The unit of measurement is a candela. It’s commonly used for reflectorized lamps and called center beam candlepower (CBCP or CP). This is a bit more specialized and is used for items like flashlights or car headlights, where the direction of light is crucial. This translates to the home particularly when you are lighting artwork.
Using various tools, from simple light meters to more sophisticated spectrophotometers, professionals can provide precise readings of these measurements. For everyday purposes, a basic understanding of lumens and footcandles is sufficient, but for tasks requiring precision, such as in photography, architecture, or horticulture, knowing the finer details of light measurement is indispensable. When choosing lighting for your home, we have more general standards to go on, but if you’re lighting a business or a school, there are many more considerations where you would want to get precise measurements.
Choosing Wisely: Evaluating Light Bulbs
The topic of choosing the right light bulb deserves a deeper dive when planning the lighting design of your spaces, than what we’ll go into right now, and will be the focus of a future article. In the meantime, I want to arm you with some basics for the next time you need to pick up some replacement bulbs.
When trying to replace a traditional bulb with an LED, here is a useful chart to guide you:
As discussed above, Kelvin tells us how cool or how warm a light appears. Go for higher Kelvins if you like it cooler, and lower for warmer tones. You may think, great! I’ll put 5000 Kelvin everywhere and I’ll feel bright, alert and happy all day everyday! It sounds reasonable, but in practice the light output doesn’t actually behave that way. Its a slightly bluish-white color and will feel cold and sterile in your living spaces. But, have you ever taken your pair of socks to the window to determine the black from a navy blue, or found a grease spot on your shirt the moment you step outsides? This 5000 Kelvin daylight color (with a high CRI) is the absolute best for seeing those slight differences in 2 different socks or that mark on your shirt. Because of that I recommend those higher kelvin bulbs in laundry rooms and closets. Just don’t forget the high CRI is just as important!
The other critical factor, the Color Rendering Index (CRI), measures how a light source displays the colors of objects faithfully, compared to natural light. A higher CRI value suggests a more ‘true-to-life’ color representation, which is essential in places like retail stores and art studios, where color discrimination is important. Equally important in closets and laundry rooms.
When evaluating light bulbs, examine both their color temperature and CRI, but also consider lifespan, energy efficiency, and cost. Advances in LED technology incorporate all of these considerations, and means that you can now have an environment-friendly bulb without sacrificing light quality. LEDs lamps do have their limitations though, check out the next article in this series for those details.
There are a myriad of decorative bulbs out there warrant consideration, and for those, choose something that resonates with you while meeting the functional needs of your space. Remember, you can always adjust your approach down the road if your needs or preferences change.
I hope this article has illuminated some aspects of light and human perception that you might not have considered before. Next time you’re replacing a bulb reflect on the science behind the scenes and how it can help create the perfect atmosphere for your activities. And stay tuned for future articles that dive deep into all of the most common lights sources for your home.