One of the most common application questions we receive from customers relates to the amount of time it takes to disinfect a surface. Accordingly, this is a critical parameter for any application design. In order to find the time required, you must first understand how we calculate the UV dose that is needed to meet the application’s disinfection goals.

**Step 1: Calculate Dosage**

For surface disinfection applications, the UV dose is the product of the irradiance and exposure time:

When it comes to systems for flowing water, requirements are determined by more complex variables. Our UVC LED requirements calculator lets you find your application needs based on such variables.

Playing with the above equation, you can find that the time necessary to disinfection a surface is equal to the quotient of the dose divided by the irradiance. Therefore, when asked how much time is required for surface disinfection in your application, you must first determine how disinfection is defined (target pathogen and log reduction). This can be found in the table in this application note (please note that all the values are collected using a low-pressure mercury lamp, and therefore need to be adjusted for the LED wavelength (and tolerances)).

**Step 2: Calculate Irradiance**

The next step is to determine the irradiance. Irradiance is defined as the power per unit area incident upon a surface at a specified distance. It is inversely proportional to the source-to-surface distance (inverse square law function). Once you know the dose requirement and calculated irradiance, it is easy to determine amount of time necessary for the given disinfection target using the above equation.

For instance, let’s consider a defined disinfection level of 99 percent reduction (i.e. 2 log reduction value) of staphylococcus aureus, and a target surface area of 10 cm x 10 cm.

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**Step 3: Calculate Time**

To determine the time required, we will determine (1) the wavelength-specific dose required, (2) the number of LEDs and relative placement/distance, and (3) minimum irradiance point on the surface.

- The dose required to achieve this level of disinfection is 5.4 mJ/cm
^{2}at 254 nm, which is an equivalent dose of 5.2 mJ/cm^{2}at 265 nm or 6.7 mJ/cm^{2}at 280 nm. - We use a single Klaran WD LED operated at 350 mA and at beginning-of-life, aligned to the surface center and placed 6 cm from the surface.

- The minimum irradiance value measured at the corner of the surface is 0.075 mW/cm
^{2}, which is approximately 25 percent of the peak irradiance value measured at the middle of the surface. This is modeled using an optical design software called Zemax, but can be estimated for a single LED, considering a point source and using the specified optical power and viewing angle.

- With this information, we can calculate the minimum time required for the whole surface to be disinfected for a 2 log reduction value of staph. aureus:

It is important to note that this time required is not absolute. It is impacted by the inputs on dose and irradiance (which is impacted by the UVC LED power, arrangement and distance to the surface).

The key point to understand is that time and irradiance are correlated. Thus, if certain parameters of a system are open (e.g. the distance between the light source and the surface, the use of more LEDs, higher drive currents) engineers can play with these variables to minimize the exposure time, or inversely increase exposure time to lower operating characteristics.

If you’re designing a product with disinfection capabilities and want to learn about the required disinfection time for your application, contact Crystal IS today to discuss your project.