Light, Color & White Balance
Light and color are seemingly familiar things, but most of us would struggle to explain them objectively. Of the two, light is a little easier, since it's a physical phenomenon that can be described by objective physical attributes. Color is more complicated, because color defines a perceived experience. Color actually "happens in the mind" and therefore depends on the light illuminating a scene, the subject of the scene, and the person observing the scene.
Light
Most people are familiar with the idea that light is made up of a combination of different pure spectral components. When we look at a rainbow, a prism, or light refracted through a diffraction grating as shown here, we see the light broken down into it's spectral components. Physicists describe these components by their wavelengths, with 380 to 400 nanometers (billionths of a meter) being about the most blue color people can see, and 700nm being about the most red color people can see. Some lights have more red or blue in them, and some are more or less continuous. Some lamps such as fluorescent and Mercury vapor for example, emit light prominently at a few different wavelengths, while others such as tungsten or Xenon flashes, emit light broadly and more evenly across the spectrum. Fortunately for us, modern digital cameras can readily compensate for these variations. To do so, the camera must first determine the "color" of the light and compensate for it, because, like people, they don't record the entire spectrum wavelength by wavelength.
Color Temperature
Light comes from many sources including the sun, a cloudy sky, light bulbs of different types, photoflash tubes, and more. In casual conversation, we often describe the differences in the colors of these sources as "warmer" or "cooler." In this context, warmer means a light with more red and less blue, while cooler means a light with more blue and less red. Paradoxically, the scientific way of describing the color of lights is exactly the opposite. Physicists (and photographers) characterize the color of light by its color temperature. When describing light this way, higher temperatures mean more blue or what we typically call "cooler" light.
Degree Kelvin color temperature definitions are based on how closely a light source matches the color a purely black object (black body radiator) would radiate (or glow) if it were heated to a particular temperature in degrees Kelvin. These colors are all on the curved-line (also known as the Plankian locus) shown on the CIE xy color diagram at the right. Since real-world light sources never match these theoretical colors exactly, they are described not by their color temperature, but by their "correlated color temperature" (CCT). The CCT points lie on straight lines that intersect the curved line. Find the straight line that goes through the point that matches the color of your light source, and that's it's CCT in degrees Kelvin.
Photographers often treat CCT temperatures as if they describe all that's needed to know about a light source's color. Someone may say "oh, that's a 5000 degree Kelvin light" and think that's all there is to say about it. For many well-behaved photographic light sources, this may be true, as their actual color lies close to the curved line on the chart, but for many real-world lights, knowing the tint, or color shift off of the curved line is also critical to accurately set white-balance either in camera or in software. Shoot-through white balance filters such as the Phoxle SpectraSnap White Balance Filter are particularly useful for helping cameras get the color right.
White Balance Filter Ideal Characteristics
Given that light sources vary in color, and cameras don't capture every wavelength of the spectrum, it's important that the camera be calibrated for the light illuminating the subject to get the most pleasing and accurate color possible. Digital cameras are getting better at automatically detecting the color of the light illuminating the scene, but they still make mistakes. The reason is that the camera does this auto white balance by analyzing the image that the camera sensor sees, and many subjects don't contain the cues (such as a clean white or gray) that help a camera determine the right white balance setting. Until cameras can recognize that the picture being taken is a close-up of a pumpkin, and know that it should be rendered as orange, there will be a need for custom white balance settings.
Custom white balance lets the photographer "tell" the camera what's white in a scene, and therefore, what the color of the light illuminating the scene is. Custom-white balance exposures can be taken of anything that is a neutral color -- that is to say, something that reflects the red, green and blue parts of the spectrum equally. Taking a picture of a white table cloth, shirt, or sheet of paper will get you close, but these are rarely perfectly neutral. Taking a picture of a reference neutral target is better, but knowing where to stand so it's not in shadow, and has exactly the right light hitting it can be tricky. For these reasons, we recommend using precisely engineered shoot-through white balance filters like the Phoxle SpectraSnap whenever possible.
The SpectraSnap, like other shoot-through white-balance filters, is placed in front of the camera lens while the camera is pointed toward the light source and the white-balance exposure is made. The keys to making a filter that enables your camera to accurately measure the color of ambient light are:
• detail-elimination – the filter should present the camera with a uniform, detail-free representation of the light in front of the camera
• uniform and accurate radial color-response – the color sensed by the camera doesn’t vary as the light source moves away from a point directly in front of the camera
• wide radial sensitivity -- the amount of light reaching the camera remains high even for light sources that are shifted away from a point directly in front of the camera
• flat spectral response – the filter should pass all visible wavelengths of light uniformly
Many products on the market claim to work well as shoot-through white-balance filters, but few achieve all of these characteristics. The SpectraSnap utilizes the patent-pending Spectrescetm technology to achieve excellent performance for all of these characteristics as shown in the images and graphs on the right.
The images to the right show the SpectraSnap's ability to eliminate detail, and provide the camera with a uniformly-exposed white balance reference exposure. Notice how uniform the picture on the right is, in spite of tremendous contrast in the original scene.
SpectraSnap typical uniform & accurate radial color response. Notice how well the SpectraSnap measured color temperature follows that of the light source up to 60 degrees off-axis. Beyond that, the shift error becomes minor because sensitivity also falls off (see next graph).
SpectraSnap typical radial sensitivity. Notice how high the sensitivity is for light sources up to +/- 30 degrees off-axis, and that there is still over 50% response for sources +/- 45 degrees off axis. This means that you don’t have to point the filter exactly at the light source, and that it will accurately measure multiple or diffuse sources.
SpectraSnap typical spectral response. Notice how flat the overall response is. This means accurate white balance for even tricky light sources. The response for the warming versions of the SpectraSnap are lower in the red end of the spectrum. The result is that the camera "turns up the volume" on these colors a bit, resulting in pictures with a slightly warmer tone that many people prefer for portraits.
Why do digital cameras have such a hard time getting the color right? And, what do those color temperature numbers really mean anyway. This is where we lay out the basics on color to help you figure it out.
There are many products on the market, but few that are truly engineered to deliver precise results in a wide range of conditions. The Phoxle SpectraSnap is, and here's where we explain the how and why.
Click here to learn more about Phoxle Flash Matching Filter characteristics, and the kind of light they produce with a range of flashes.
Both Phoxle SpectraSnap White Balance Filters and Flash Matching Filters rely on Phoxle patent pending Spectresce technology to cost-effectively deliver breakthrough accuracy and affordability.
Flash Matching Filter Characteristics
Using custom white balance and a shoot-through filter with the characteristics described above will produce pleasing and accurate color in a wide range of circumstances. One common situation where it often won't is when fill flash is used. The issue is that the color of the light from the flash and the color of the ambient light don't match, so which one is the camera supposed to adjust for? The solution is to modify the color of the light sources to match each other, and the easiest one to modify is the on-camera or camera-triggered flash.
Phoxle Flash Matching Filters bring new convenience to the task of matching the color of a camera's flash to the color of ambient light. They come in sets designed to shift a 6500K light source to various CCT's from 3000K to 10,000K. They are also unique in that they are backed with a low-tack reusable adhesive, so you simply peel the desired filter from their booklet, and apply them over your flash.
The essential characteristics of Flash Matching Filters are:
* Predictable color temperature shifts of typical flash output correlated color temperature outputs, while producing light colors that lie near the Plankian locus for ideal black-body radiators.
* Spectral transmission characteristics that are free from large spikes or dips
* And relatively low loss for the highest portion of the spectral response curve.
As can be seen from the graphs below, Phoxle Flash Match Filters have excellent spectral characteristics and shift the color of light from a photo flash to predictable points on the Plankian locus -- the ideal points for a tint-free light.
Light + Subject + Observer
= Color
An extreme example of the kind of image digital cameras have trouble auto-white-balancing
Typical detail eliminating ability of the Phoxle SpectraSnap: left is without the filter, and right is with
The Phoxle SpectraSnap White Balance Filter
with patent pending Spectresce technology
Spectresce Technogy
Spectresce TM is a coined name for Phoxle proprietary and patent pending technology that literally creates spectral response characteristics necessary for manufacturing precision optical components such as the Phoxle SpectraSnap White Balance Filter and the Phoxle Flash Matching Filters.
In practice, what Spectresce technology allows the engineers at Phoxle to do is quickly design and fabricate optical components with precision spectral characteristics. Both the Phoxle SpectraSnap and Flash Matching filters have an optical coating on their front surface that's been applied using the Spectresce technology. This layer precisely controls the amount of light of different wavelengths that passes through the filter.
In the case of the SpectraSnap, the filter contains myriad tiny beads that produce the diffusion necessary to obscure detail in the image, but this also introduces a red shift in the same way that particles in the atmosphere produce red sunsets. To neutralize this red shift, and provide accurate white balance reference images, the Spectresce coating on the surface absorbs more red light than green or blue, which results in the extremely flat spectral response shown above.
In the case of the Phoxle Flash Matching Filters, the Spectresce technology enabled creation of a family of filters with precise spectral characteristics that produce a tint-free color temperature shift (also shown in graphs above).
We've only begun to take full advantage of the Spectresce technology, and expect to find many new applications for it in the years to come.
