PD300RM-UVA: Ophir’s New Irradiance and Dosage Sensor

A new member recently joined Ophir’s irradiance and dosage sensor family – PD300RM-UVA, and it has some important new features compared to its older brothers. In this blog post we will discuss its benefits and advantages for certain applications, such as measuring broadband LED sources.

First we consider its design – it has a 2.75 mm aperture that allows better mapping of the irradiance uniformity of wide beams and also permits measuring higher irradiance and dosage levels than its siblings (up to 15 W/cm²). Due to a unique diffuser geometry its cosine correction is comparable to sensors with much larger apertures.

The sensor is not black anodized as its other family members, but rather keeps its original metal color to reduce light absorption in the sensor’s body – making more resistant to effects of UV . It is also equipped with a UV-resistant cable that connects the sensor’s head to one of the supported Ophir meters (StarLite, StarBright, Juno+).

Now let’s focus on the main feature of the new sensor – flat spectral response in the wavelength range of 350 – 450 nm. Why is that so important? Irradiance and dosage sensors are typically based on photodiodes as the light absorbing element.
A photodiode’s responsivity changes as a function of the light’s wavelength, so in order to accurately measure the power or irradiance levels of light sources the user must enter the light’s wavelength into the meter to get a calibrated reading.

But what happens if the exact wavelength is unknown, or changes with time due to thermal shifts? There is even a bigger problem when measuring broadband light sources such as LEDs, which can have a full-width-half-maximum bandwidth of 10-20 nm: in this case there isn’t a single wavelength (and therefore there isn’t a single calibration factor) that that characterizes the light spectrum, so accurate measurement are impossible with regular irradiance sensors.

Now that we agree that there are times where accurate irradiance measurement is needed without knowing the exact light properties, let’s discuss the solution: since the problem arises due to the responsivity curve of the photodiode, adding another optical element that will flatten the total responsivity of the sensor will solve it. How? If we’ll ensure that the electrical signal the photodiode outputs does not depend on the light’s wavelength, we will not need to know the exact wavelength in order to accurately measure the light source under investigation.

The optical element Ophir’s team designed for this purpose is a custom-made filter with unique transmission properties, inversely proportional to the photodiode’s responsivity curve.

Thus, when placed together inside the sensor, the result is a flat response in the wavelength range 350 – 450 nm. To further improve the flatness we divided this range into two: UV (350-400 nm) and VIS (400-450 nm), easily selected in the attached meter. In this way, broadband light sources can be easily measured by the new sensor, which also allows measuring different light sources simultaneously.

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