Measuring UV adhesive cure means measuring two numbers. Irradiance (the industry often says UV illuminance or UV intensity; unit mW/cm²) is how strong the light is on the workpiece surface right now. Accumulated energy (energy density or dose; unit mJ/cm²) is the total amount received over a period of time. 1 mW/cm² for 1 second = 1 mJ/cm². The instrument that measures these two numbers is called a UV radiometer (in Chinese, UV energy meter or UV illuminance meter); an ordinary lux meter that measures how bright a lamp is cannot be used to measure UV.
Sources: Incure, “What Is a UV Radiometer and Do You Need One?”; Quantum Design Europe, “UV-curing applications: Understanding the specifications of light measurement devices – Part 2”; GoPhotonics, “UV Radiometer”.
This article is divided into 5 stages: ① tell the units apart ② why an ordinary lux meter cannot measure UV ③ choose the wavelength ④ how to measure ⑤ recording and control. This is the general case; for cure conditions, follow the adhesive’s technical data sheet (TDS).
Stage ①: First tell irradiance and energy apart
| What is measured | English | Common units | Meaning |
|---|---|---|---|
| Irradiance (UV illuminance, UV intensity) | irradiance | mW/cm², W/cm² | The UV power falling on each square centimeter of the workpiece surface right now |
| Accumulated energy | energy density, dose | mJ/cm², J/cm² | Irradiance multiplied by exposure time, the total received |
Energy = irradiance × time. For example, 100 mW/cm² for 3 seconds gives an accumulated energy of 300 mJ/cm² (numbers are illustrative). If the adhesive’s TDS states energy, the energy you measure has to match it; if it states irradiance, look at irradiance.
Sources: Quantum Design Europe, “UV-curing applications: Understanding the specifications of light measurement devices – Part 2” (dose = W/cm² × seconds); Incure, “What Is a UV Radiometer and Do You Need One?” (irradiance in mW/cm², integrated to give mJ/cm²).
Stage ②: Why an ordinary lux meter cannot measure UV
An ordinary lux meter measures visible light, and its reading is weighted by how sensitive the human eye is at each wavelength (the CIE visual curve); UV is outside the range the human eye can see, so a lux meter cannot read it, or reads it inaccurately. To measure UV you need an instrument for radiometric quantities, that is, a UV radiometer or UV irradiance meter marked for the UV band.
Sources: Konica Minolta Sensing, “Radiometers and Photometers” (a photometer measures visible light and its spectral response follows the CIE visual curve; in industry a radiometer is used to measure UV and infrared); Wikipedia, “Photometry (optics)” (photometric quantities are weighted by the sensitivity of the human eye, while radiometric quantities are not limited to visible light and include UV).
Stage ③: Choose the wavelength — the sensor head must match the lamp
UV lamps include broadband mercury lamps and single-wavelength UV LEDs such as 365, 385, 395 and 405 nm. A radiometer made for broadband, used to measure a narrow-wavelength LED, may read too high or too low; when measuring an LED, use a radiometer or sensor head calibrated for that wavelength.
| Lamp type | Wavelength | What to look at when choosing a radiometer |
|---|---|---|
| Mercury lamp (broadband) | Several bands: UVA, UVB, UVC, UVV | The band marked on the radiometer must cover the band the adhesive needs |
| UV LED | Single wavelength such as 365, 385, 395, 405 nm | Choose a model calibrated for that wavelength |
Sources: Incure, “What Is a UV Radiometer and Do You Need One?” (a broadband probe covers about 250–400 nm; an LED needs a radiometer calibrated for its wavelength); Ophir (MKS), “Measuring Broadband UVA And Violet Led Light Sources” (when a sensor’s spectral response is not calibrated for the LED wavelength, the reading deviates); Quantum Design Europe, “UV-curing applications: Understanding the specifications of light measurement devices – Part 2”.
Stage ④: How to measure
When measuring, put the radiometer where the workpiece will really be, and send it through the process the same way the workpiece goes; do not just measure casually on the workbench. If the fixture is changed, the working distance may change with it; after changing a lamp or a light guide, measure again. On a conveyor line, use a radiometer that can record data and let it travel through the line with the product, showing how irradiance changes over time and the accumulated energy.
Sources: Incure, “What Is a UV Radiometer and Do You Need One?” (measure irradiance at the production working distance; modifying a fixture may change the working distance; verify after changing a lamp or replacing a light guide; use a data-logging radiometer on a conveyor); EIT, “UV PowerMAP Plus” data sheet (the radiometer goes into a static or conveyor UV process to obtain data, the data corresponds to the energy and irradiance the real workpiece receives, and total energy density in J/cm² is listed); Quantum Design, “UV-CureRight Radiometer” product page (the radiometer travels through the line on the conveyor and shows the irradiance curve and the integrated dose in J/cm²).
Stage ⑤: Recording and control
A lamp or LED loses output as it ages; it may look lit while the amount actually reaching the part is no longer enough. Measuring irradiance on a fixed schedule lets you find that the lamp has decayed before cure quality goes wrong. At what reading the lamp should be replaced follows the lamp and adhesive makers’ specifications; this article gives no fixed percentage or number of days.
Sources: Incure, “What Is a UV Radiometer and Do You Need One?” (measure irradiance on a fixed schedule to detect lamp decay); Quantum Design Europe, “UV-curing applications: Understanding the specifications of light measurement devices – Part 2” (lamp intensity falls with use time).
Frequently asked questions
Are a UV energy meter and a UV illuminance meter the same kind of instrument?
Both are UV radiometers; each maker names them differently (energy meter, illuminance meter, radiometer). Before buying, look at whether the specification gives both readings, irradiance and energy.
Sources: Incure, “What Is a UV Radiometer and Do You Need One?”; GoPhotonics, “UV Radiometer”.
Can an ordinary lux meter be used to measure UV?
No. An ordinary lux meter is weighted by the human visual curve and measures visible light; UV cannot be read, or is read inaccurately. Use a radiometer marked for the UV band.
Sources: Konica Minolta Sensing, “Radiometers and Photometers”; Wikipedia, “Photometry (optics)”.
How much UV does an adhesive need to cure?
It follows the adhesive’s TDS. Different UV adhesives have different cure conditions, so take the irradiance, energy and wavelength given by the adhesive maker as the reference, and then use a radiometer to confirm the amount the line actually delivers.
Sources: Quantum Design Europe, “UV-curing applications: Understanding the specifications of light measurement devices – Part 2”; Incure, “What Is a UV Radiometer and Do You Need One?”.
Further reading and related products
For a conformal coating that needs UV cure after application, see How to Choose, Apply and Inspect Conformal Coating; for the cure and release of flip-chip underfill, see Flip-Chip Underfill in Volume Production.
To choose a UV radiometer, the product page groups models by mercury lamp or UV LED wavelength and by whether you need to measure irradiance or energy; for models and prices see UV Energy Meter (UV Radiometer, UV Illuminance Meter).
Notes: Scope and exceptions
This article covers how to measure irradiance and energy when curing UV adhesive and how to choose a radiometer; the measuring range, accuracy and calibration method of each radiometer follow each model’s data sheet. This is the general case and still depends on the individual case.