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Incident-light meters. Incident-light meters measure the amount of light falling on the subject and are only available as off-camera, handheld devices. The most prominent feature of incident-light meters is the translucent white hemispherical dome (the “lumisphere”) that both encloses and provides even illumination to the photocell within, which measures the light’s intensity. To obtain an exposure reading, you hold the incident-light meter at the position of the subject, ensuring that the dome is in the same light, point the lumisphere towards the camera, and take a measurement.
Every digital camera that features either exposure compensation or a manual exposure mode will feature an exposure meter either in the viewfinder, the top LCD panel, or both. The exposure meter is a simple linear scale of relative exposure values marked by notches, dots, or numbers, and on some cameras, all of the above. The standard exposure index, characterized by the big central notch, or a zero, always represents the optimal exposure as determined by the camera’s programming. Flanking the standard exposure index are marks that represent increments of 1/3 EV and 1 EV. The marks on the right side represent added (or positive) exposure, and the marks on the left represent subtracted (or negative) exposure. The exposure level indicator is the needle or marker beneath the notched scale.
Every reflected-light meter is calibrated to provide an exposure reading that renders the subject it’s pointed at as middle grey. If you take a reflected-light reading off a black square of paper, the meter will propose exposure values that will render the paper middle grey in the photograph, thereby overexposing it. If you take a reflected-light reading off a white square of paper, the meter will propose exposure values that will render the paper middle grey in the photograph, thereby underexposing it. The only tone for which reflected-light meters provide objectively accurate exposure values is middle grey.
Evaluative/Matrix/Multi metering. This is the default and most sophisticated metering mode on most DSLRs and mirrorless cameras because it works in a variety of situations and provides accurate results most of the time. It works by dividing the frame into multiple metering segments and analyzes their brightness and, sometimes, colour. The resulting matrix of multiple segments is evaluated based on composition, colour, and distribution of tones. Nikon cameras that feature 3D matrix metering also factor the distance information provided by the autofocus modules. Some mirrorless cameras evaluate the scene for the presence of prominent faces.
The team installed 222 nm UV sanitizers in the bathroom along with a suite of chemical sensors. “I've never installed state-of-the-art air quality measuring devices in a bathroom,” said Link, first author on the study and patron of the laboratory lavatory. “We had to figure out how to get enough power into the bathroom for these instruments. We had to make sure we had a wireless internet connection so we could monitor our instruments in real time. We even had to consider things like not dropping electrical cords into the toilet.”
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Paper: Michael F. Link, Rileigh L. Robertson, Andrew Shore, Behrang H. Hamadani, Christina E. Cecelski and Dustin G. Poppendieck. Ozone generation and chemistry from 222 nm germicidal ultraviolet light in a fragrant restroom. Environmental Science: Processes & Impacts. Published online May 20, 2024. DOI: 10.1039/D4EM00144C
But this wavelength has its own unique drawback: It turns oxygen in the air into ozone. Ozone itself can be concerning, and it is also highly reactive with other chemicals in the air and creates new secondary compounds. Link and Poppendieck wanted to know whether the new chemicals could pose a health risk.
For most people, a smelly bathroom is something to avoid. But when the ventilation stopped working in his lab’s hallway bathroom, indoor air expert Michael Link smelled opportunity.
At the National Institute of Standards and Technology (NIST), Link and his colleague Dustin Poppendieck study air sanitizers that use ultraviolet light (UV). These devices kill pathogens such as those that cause the flu and COVID-19, but they can also cause unintended chemical reactions in the air.
Together, the exposure meter scale and exposure level indicator serve one of three functions depending on the exposure mode you’re using.
When the COVID-19 pandemic began, people started looking for effective ways to control airborne disease in indoor spaces. One promising approach was to use UV radiation. UV is light, but with a wavelength too short for our eyes to detect. Red light has a wavelength of about 700 nanometers (nm); yellow light, 580 nm; and ultraviolet light is between 400 and 100 nms. The shorter the wavelength, the higher the energy. UV light has enough energy to break apart DNA, which is why it’s effective at killing viruses in the air (and why you should wear sunscreen at the beach).
The only thing a light meter can do is measure the actual brightness of an object; it can make no assessment of whether the object is light or dark, in deep shade or in bright sun. It’s up to you to evaluate and interpret the nature of your subject according to the information provided by the meter. By understanding how light meters work and are calibrated, you’ll be better able to use them to determine the appropriate exposure for a scene.
Reflected-light meters. Reflected-light meters work by measuring the light reflected off the subject, thereby measuring the subject’s brightness. Reflected-light measurements are taken from the intended position of the camera regardless of whether the meter is on- or off-camera. Virtually every digital camera meters exposure using reflected-light entering the lens, which is known as through-the-lens (TTL) metering.
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Spot metering. The spot meters found in cameras are similar to those in handheld light meters. They read light from a small central area that typically comprises one to five percent of the frame. On Nikon and most mirrorless cameras, the spot meter overlaps with the active focus point. On Canon cameras, the spot meter is always found in the centre of the frame. Spot metering is both incredibly empowering and finicky. When using any of the auto-exposure modes with spot metering, the camera will set its exposure values to produce a middle grey rendition of whatever subject the spot is pointed towards, and all other tones present in the scene will align according to their relative brightness. For this reason, using spot metering in any of the auto-exposure modes will produce wildly inconsistent exposures of diverse subjects under the same lighting. For example, when using auto-exposure, spot metering a black dog will produce an exposure where the dog appears grey and the scene is overexposed; spot metering a white dog will produce an exposure where the dog appears grey, and the picture is underexposed; and, spot metering a grey dog will yield an exposure where the dog appears grey, and the scene is correctly exposed. Ponder this point.
The 18-percent, or middle grey, standard is the mathematical average of all tones evenly distributed across a scale from absolute black to absolute white. In digital photography, the middle grey tone lies at the precise middle of the luminance histogram, and there are corresponding middle tones for the red, green, and blue colour channels. (All future references to 18-percent grey or middle grey refer to the tone independent of its colour). When printed, a middle grey tone reflects 18 percent of incident light (i.e., light falling upon it). Photographic exposures derived from metering the average of all tones in an average scene are remarkable at obtaining average results.
Reflected-light metering is how cameras measure subject brightness to determine optimal exposure. Most digital cameras offer several metering modes that feature varying degrees of sophistication. Regardless of the metering mode, it’s important to remember that every reflected-light meter, including your camera’s, is calibrated to provide an exposure reading that renders the subject as a middle grey tone. The most significant difference between the metering modes is in their method of interpreting the distribution and variation of the scene’s tonal range.
After analyzing their results collected over two weekends — one with ventilation and one without — they found that the 222 nm UV light was producing enough ozone to react with chemicals in the air. The byproducts included formaldehyde, volatile organic compounds and nanoparticles.
Handheld reflected-light meters come in two broad varieties. Wide-angle reflected-light meters take an average reading from across a relatively large area. Spot meters are reflected-light meters that read the brightness from a relatively small portion of the scene—typically one degree of your field of view. All handheld spot meters feature a magnified viewfinder with a clearly marked circle that outlines the metering zone. Spot meters tend to be more expensive than both incident- and wide-angle reflected-light meters.
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Average metering. This mode measures the average brightness of the entire frame and sets exposure to yield a middle grey tone rendering of that average. This mode produces particularly accurate results for landscapes with the sun outside the frame. It provides relatively consistent exposures across multiple shots of different subjects under the same lighting.
Since incident-light meters measure the intensity of light striking the subject, they provide accurate exposure information regardless of your subject’s inherent brightness; so long as the subjects in your scene are evenly illuminated by the same source of light, dark tones will be rendered as dark, grey tones as grey, and light tones as light. Furthermore, incident-light meters are incredibly useful for making accurate measurements in a controlled-lighting environment, such as a studio, and are especially practical when used to calculate precise contrast ratios between different lights.
Auto-exposure bracketing range. Exposure bracketing is the technique of taking multiple exposures of the same scene while slightly varying the amount of exposure between the individual photos. This technique is incredibly useful when capturing subjects with complex lighting, or when you intend to combine multiple exposures into one high dynamic range (HDR) image. Auto-exposure bracketing (AEB) automates the process within a range defined by the user. When AEB is activated, the camera’s exposure meter will show multiple exposure level indicators specifying the AEB range of the consecutive shots.
But there’s a drawback to this high-energy UV; it can cause sunburn or temporary blindness. It has to be directed away from people, while still shining on the air they breathe. To use this UV safely, installers must position the lights carefully within ventilation shafts or point them toward the ceiling. This challenge of positioning UV sources is one of the main reasons this form of air purification isn’t used more widely. To avoid that risk, there has been a lot of interest in using less-damaging UV light.
NIST indoor air quality researchers closed a men’s restroom over two weekends to measure indoor air pollution caused by UV air sanitizers.
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Despite the neutrality of incident-light measurements, there are several inherent disadvantages you should know about. First, measuring the intensity of light falling upon a distant subject may be difficult, impractical, or impossible. For instance, an incident reading of the night sky is impossible, of a sunlit mountain from an outlook covered by the canopy of trees is impractical, and of a subject under complex or mottled lighting is imprecise. Secondly, since incident-light meters measure subject illumination, the exposure values they provide for the aperture are in theoretical f‑stops. If the transmission value of your lens is significantly different from the theoretical f‑number it’s set to, the exposures won’t be accurate (see F‑Stops and T‑Stops).
UV air sanitizers were first widely used in the 1950s to combat tuberculosis. Those early attempts used UV with a wavelength of 254 nm, because that energy level is the most effective at breaking apart DNA and killing pathogens. It is still the most common type of UV air sanitation.
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Exposure level indicator. In Manual mode, the exposure level indicator shows how much your set exposure values deviate from the levels the camera believes are optimal, which always reside at the standard exposure index. (See Manual Exposure).
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A light meter is a tool that measures light intensity and aids photographers in setting the appropriate exposure. Photographic light meters are available as off-camera handheld devices, and they’re built into virtually every digital camera.
They decided to set up their study in a men’s restroom because it had urinal screens — those brightly colored pads placed over the drain to prevent splashing. These screens release fragrances (to mask the odor of urine) that include terpenes, a group of chemicals that reacts strongly with ozone. This study primarily focused on terpenes, but most bathrooms have air filled with chemicals such as cleaners and other scented hygiene products that would also react with ozone.
Ultimately these studies could be used to develop tests for UV air purifiers and guidelines for how to minimize their production of volatile chemicals. This information could ultimately guide decisions on how to best deploy UV sanitizers in a wide range of settings to optimize their benefits to human health.
To fully understand the importance of these chemical byproducts, researchers will need to conduct additional studies to determine how dangerous these pollutants might be, including on a wide variety of indoor spaces and environments that could benefit from the use of UV sanitizers. Other environments are expected to create different kinds of chemicals at different rates, and ventilation could affect how long they will stick around in the air.
Centre-weighted metering. This metering mode is designed primarily for portrait photography. It considers the average brightness of the entire frame but gives the greatest prominence to the tones in the central region. Centre-weighted metering isn’t recommended for scenarios in which multiple shots of various subjects are anticipated, as the exposures will fluctuate.
Link and Poppendieck wanted to know what those reactions looked like in the complex chemical soup of a real restroom. Their latest research, published in Environmental Science: Processes & Impacts, shows that these reactions produce pollutants that could be a health risk. As these sanitizers make their way into our homes and buildings, this research could help guide decisions on using them.
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In the carefully controlled environment of a lab, researchers can precisely measure air chemistry. But people don’t live in stainless steel lab chambers. “Standard lab tests only measure how ozone reacts with one or two pure chemicals,” Poppendieck explained. “In the real world, indoor air has tens if not hundreds of different chemicals that could potentially react in significant concentrations. Michael Link and I talked it over, and he suggested we run a real-world test to help us understand whether these lights are safe.”
Exposure compensation amount. Exposure compensation allows you to increase or decrease image brightness beyond what the camera determines is optimal exposure. (See Exposure Compensation). When the camera is set to the Program, Shutter Priority, or Aperture Priority modes, the exposure meter shows the amount of compensation applied in units of EV. When the exposure level indicator is set to zero, there’s no exposure compensation, and the camera reverts to its standard exposure programming.
UV light with a wavelength of 222 nm is still effective at killing pathogens, but the shorter wavelength doesn’t penetrate as deeply into human skin, so it’s safe to shine directly on people. Because of this, 222 nm UV light could be used to sanitize the air in small, crowded places like classrooms, elevators and public transportation, places where 254 nm UV sanitizers would be difficult to install easily.
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