Posts Tagged ‘spectrum’

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Tungsten vs. Fluorescent vs. LED lights: Light Quality (CRI) and Conclusions (Post #4 of 4)

Monday, March 4th, 2013

This post is the last of a series of four comparing the advantages and disadvantages of different types of continuous lights for photo and video work. Here are the previous ones:

Post #1: Choosing Lights: Tungsten vs. Fluorescent vs. LED
Post #2: Efficiency (i.e., power usage) and Heat Generation
Post #3: Portability and Speed of Setup & Ease of Use

Light Quality (CRI)

When choosing a light source, the final (and perhaps most important) issue is the quality of the light that that light source emits; in other words, its spectrum, or “CRI.” As most people know, light is made up of a whole spectrum of wavelengths, which results in different colors (in terms of visible light, red light is at one end of the visible spectrum with long wavelengths and blue is on the other end, with short wavelengths). Different types of light sources (LED, fluorescent, tungsten incandescent, HMI, the sun) emit different mixtures of wavelengths of light, and the best ones, like the sun, emit a nice, broad, even spectrum of wavelengths (without large spikes or dips at any given wavelength), which allows objects of different colors to appear as vibrant as possible in a photograph or video.

Color Rendering Index (or “CRI”) is a measure of the mix of spectrum that a light emits. This is the biggest advantage of the traditional xenon and tungsten lights: they emit the broadest spectrum of light of any of the types of photo and video lights here (this is because they closely mimic the behavior of a concept known to physicists as “black body radiation.” If physics is your thing or you really want to understand this subject in depth, read about black body radiation and Planck’s law). As a result, tungsten and xenon lights have the highest CRI (100, or close to it). LED and fluorescent lights, on the other hand, emit light that contains more of certain wavelengths and less of others (uneven spikes and dips), and therefore have lower CRIs (from as high as 95+ for good quality lights to as low as 60-70 for low-quality lights). The effect of lower CRIs is that some colors, including skin tones, can appear muted, washed out or unnatural in photos and videos. For this reason it is essential to choose lights with high CRI values.

Both my LED panels and my fluorescent lights have CRI values above 90. That is, they emit a quite broad spectrum of light that will illuminate objects of all colors well (that is not to say that they are accurately color balanced or white balanced for any particular target: CRI and white balance are two completely separate issues… more on that in tomorrow’s post). To illustrate the broad spectrum of light from each of these sources, take a look at the images below.

Emission Spectra ©2013 Chris Conti Photography

The emission spectra of my 1x1 LED panel (left) and fluorescent lights (right). Both show good, broad, consistent output. ©2013 Chris Conti Photography

To make the images above, I projected near-parallel beams of light from my LED (left) and fluorescent (right) light heads into an optical prism, which refracted the light into its constituent wavelengths, and photographed the results. Note that all of the colors of the visible light spectrum are well-represented. This is an indication of the high CRI value of these lights.

 

Conclusions

After experimenting with and testing various lights both in theoretical tests like the emission spectra above as well as practical ones like lighting a white seamless with them, using them for portraits, etc., I’ve made a few conclusions. In no particular order, here goes:

- The concern that LED and fluorescent lights emit poor-quality light as compared to tungsten lights is unfounded. With a tiny bit of adjustment via gels (more on that tomorrow) both from a subjective standpoint (how they look) and a technical standpoint (technical measures of their light emission), these lights look great.

- Both LED and fluorescent lights consume a tiny fraction of the amount of power that tungsten lights do (which makes them more usable in the field), and don’t generate the searing heat of tungstens (which is always inconvenient and can be destructive and painful, and uncomfortable for subjects).

- On the other hand, fluorescents and LEDs don’t generally generate the quantity of light that most tungsten heads do, so it may be necessary to use more of them for certain applications (flooding a white seamless, much less a full cyc wall, requires a huge sheer quantity of light), so these lights might not be terribly well-suited for these applications.

- Fluorescent lights are much less portable and more time-consuming to set up than tungsten lights, but LEDs are easier and faster.

- Light modifiers and accessories like softboxes don’t really exist for LED panels yet (although I did just make a softbox for my 1×1 LED… perhaps that’ll be a future blog post…), but fluorescent heads can usually easily accommodate anything mounted on a standard speed ring.

So what does it all mean? These lights are tools (just like all of our other kinds of gear), and they each have advantages and disadvantages, and are better suited for some tasks and worse for others, and the right tool for the job will depend on the particular job: lighting a large stage with a two-wall cyc wall is still best done with high-power, high-output tungsten or HMI lights. A quick location interview is probably best done with a couple of LED panels. For a small- to medium-sized studio shoot, fluorescents are probably the best bet.

Tomorrow I’m going to be going to a location and shooting in the same room all day. What will I bring? Fluorescents and a couple of LEDs.

I’ve learned a lot experimenting with all these different types of lights. If you’ve read this blog post and the few that came before it, hopefully I was able to share some of that with you. If you have any comments, different opinions or questions, let me know!

-Chris

P.S.- Since I’m going to be using a mixed bag of different light sources that result in a mixed bag of  color temperatures, in order to work well together the lights need to be balanced to each other. Balancing my mixed bag of lights will be the subject of tomorrow’s post…

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