How to Choose a UV Light Meter — UVA, UVB and UVC Meters

How to Choose a UV Light Meter — Lutron UVA, UVB and UVC Meters

Ultraviolet meters are not interchangeable. Each one is built around a sensor that responds to a narrow slice of the spectrum, and a meter pointed at the wrong kind of lamp reads near zero — which looks exactly like a dead lamp. The single most common mistake is buying a UVA meter to check a germicidal lamp, or the reverse.

This page sorts the choice out in one step. Find the lamp you are measuring, and it tells you which meter reads it.

Start Here: What Are You Measuring?

A UVA meter will not read a germicidal lamp, and a UVC meter will not read a blacklight or a curing lamp. The bands do not overlap. If the reading is near zero on a lamp that is plainly running, check the meter's band before you condemn the lamp.

The Three Ultraviolet Bands

• UVA, 315 to 400 nm. Long wave. Blacklights, curing lamps, inspection lamps, tanning equipment, insect traps. Peak interest is 365 nm.
• UVB, 280 to 315 nm. Medium wave. Weathering and degradation testing, phototherapy, some laboratory work.
• UVC, 100 to 280 nm. Short wave. Germicidal disinfection, almost always at 254 nm. Does not reach the ground from sunlight, so any UVC you measure came from a lamp.

Comparison

All six use a cosine corrected sensor on a separate corded probe, so the sensor goes into the beam while you read the display from a safe distance. Ultraviolet accuracy is ± (4% F.S. + 2 digits) on every model except the UVC-254A, which is ± (2% F.S. + 2 digits).

Choosing Between Similar Models

Do you need data logging?

The SD models — UVA-365SD, UVC-254SD and YK-37UVSD — write every reading to an SD card with a date and time stamp, at intervals from 1 second to 1 hour, and the file opens directly in Excel with no software to install. Choose one if you have to document lamp output rather than simply observe it: periodic validation of a disinfection system, a curing process that must be shown to have been within specification, or a weathering chamber monitored over days.

If you are spot-checking lamps and writing the number on a clipboard, the non-logging models do the same measurement for less.

Micro-watts or milli-watts?

The UV-340A reads in µW/cm² and tops out at 19,990 µW/cm², which is about 20 mW/cm² — the same ceiling as the mW models, with finer resolution at the bottom. It suits weaker sources and hazard surveys. The mW models suit production curing heads and anything strong.

The UVC-254A is the widest of the lot, with three ranges from 0.1 µW/cm² resolution up to 19.99 mW/cm². That bottom range is what lets it survey UVC leakage around an enclosure — measuring what escapes to where a person stands, not just what the lamp puts out.

One band or two?

The YK-37UVSD ships with both a UVA probe and a UVC probe. If your work spans both — a facility with disinfection equipment and curing equipment, a laboratory with transilluminators and biosafety cabinets, a service company covering several customers — it replaces two instruments and two calibration schedules.

What Every UV Measurement Needs

• Record the distance. Irradiance falls off sharply as the sensor moves away from the lamp. A reading without a stated distance cannot be compared to anything, including itself next month.
• Let the lamp warm up. Most ultraviolet sources take several minutes to reach stable output, and metal halide lamps considerably longer from cold.
• Hold the sensor square to the source, or flat on the surface being evaluated. A cosine corrected sensor handles angled light properly, but one aimed off to the side of the beam is measuring a different part of the field.
• Know the difference between irradiance and dose. The meter reads irradiance in W/cm². Germicidal and curing effect depend on dose, which is irradiance multiplied by exposure time. Since 1 mW equals 1 mJ per second, 0.5 mW/cm² for 60 seconds delivers 30 mJ/cm².
• Store the sensor dry. Every one of these sensors loses gain if stored damp. Seal the probe in a bag with a desiccant pack between uses and replace the desiccant periodically — it is the single biggest factor in how long the calibration holds.
• Calibrate annually. Sensor output typically drifts about 1% in the first year. A one year interval is the usual recommendation.

Ultraviolet Safety

Ultraviolet light injures eyes and skin, and it gives no warning while it is happening. Pain and redness appear hours after the exposure, by which time the injury is done — so exposure can never be judged by how it feels. UVC at 254 nm is the most hazardous of the three bands, but high output curing and exposure lamps are intense sources too.

• Never look at an energized ultraviolet lamp, even briefly, and even through glass or a reflection.
• Wear ultraviolet rated eye and face protection, and cover exposed skin. Ordinary safety glasses are not necessarily UV rated.
• Use the corded probe the way it is meant to be used: the sensor goes in the beam, you stay out of it.
• Do not judge output by visible brightness. Much of this light is invisible, and a lamp that looks dim can be a strong source.
• Some germicidal lamps generate ozone. Ventilate accordingly.
• Follow the exposure limits set by the equipment manufacturer and by your workplace.

Measuring Visible Light Instead

If what you need to measure is light you can see — office and task lighting, a classroom, a greenhouse canopy, a photographic set, a parking lot — that is a different instrument. A light meter reads illuminance in lux or foot-candles, weighted to match the response of the human eye, and it will not tell you anything useful about a UV source. Likewise a UV meter tells you nothing about whether a room is bright enough to work in.

One foot-candle is approximately 10.76 lux. Foot-candles are the usual unit in the United States; lux nearly everywhere else. Most of these meters read both.

• LX-113S — auto ranging to 50,000 lux, separate corded probe, lux or foot-candles.
• LX-114S — LED, lux and foot-candle meter with remote sensor.
• LX-1148SD — LED, lux and foot-candle meter with SD card data logging.
• LX-103 — digital light meter, three ranges, lux and foot-candles.
• LX-105 — three ranges to 50,000 lux, remote sensor.
• LM-81LX — basic pocket light meter, lux or foot-candles.

A note on LED lighting: meters marked for LED use are corrected for the spectral output of LED sources, which differs enough from incandescent and fluorescent light to matter. If the installation you are measuring is LED, choose one of those.

Questions

If you are not certain which meter fits your work, tell us what lamp you are measuring and what the reading is for, and we will point you at the right one. Use the Contact Us form at inspectusa.com/contact.