Matthew Oliver Profile picture
Dec 8, 2023 26 tweets 7 min read Read on X
Some concerns starting with use of far uv "skin safe" lights. I expect the exposure limits will be increased again with research, but right now it's possible to blow through those limits with a light of any power (say 10 or 12 watts and up).

Calculating incident energy and
exposure can be easy, but it can also be complicated.

I can't teach this in a thread, but will try to illuminate the complexities involved.

A simple light source that emits constant energy in all directions it illuminates can be treated as an isotropic source.
That's not exactly isotropic, but works for a bulb in a case that shines in one direction. Might call it 'bounded isotropic'. Anyway, if the incident energy is everywhere similar, you can treat exposure in that area like it's isotropic. Like the Nukit.

Means inverse square law Nukit light source far uv 3 watts by Naomi Wu
will work.

How do you know if your light is isotropic within the beam? Manufacturer should tell you, or you can measure it, or look at the physical structure of the light source.

This is where the complicated bit starts. This is not engineering advice

en.wikipedia.org/wiki/Inverse-s…
The physical structure of the source determines the energy pattern produced. Asymmetric structures do not usually produce symmetric energy distributions unless designed that way. These excimer lights are mostly asymmetric.

Means that tube puts out different energy red to blue Image
Imagine you're starting at the surface of the light box, then arcing in a semi-circle overtop, back to the surface. Do that for the red line, then the blue, and measure incident energy.

If you plot that energy on a polar graph, it will give you the light energy pattern. Image
This must be measured in both 'cuts' blue and red, otherwise you may miss asymmetries. This light (the UVCAN Delphi Industrial 20W) is actually pretty symmetrical in both the red and blue cuts.

The graph below shows normalized energy as a function of angle in degrees. Image
Beamwidth is 64.1 degrees. Light is pointing up at 0 degrees.

For this light, if you knew the peak power at 0 degrees for a given distance, you could use inverse square to calculate approximate incident energy anywhere in that about +/- 15 degree conical area. A polar plot of the Delphi Industrial incident energy in two cuts 90 degrees apart
The reason you can use inverse sq as an ok approx in that +/- 15 degrees, is because you can treat the light energy in that conic area as isotropic. You know that will work because you have the red and blue energy patterns from the manufacturer.

That light is the type I might
do the math to ceiling mount above a dining table to flood the space over the table with enough energy to get a high degree of pathogen inactivation. The relatively narrow beamwidth means people sitting alongside the table will be at lower incident energy. Can calculate that
with trig, and control the width and incident energy by varying the light's height above the table.

That's not a particularly complex design challenge. Couple hours with Excel and a tape measure, could scope things out pretty well and be confident your dinner guests won't burn
Now it gets trickier. That Delphi has a semi-circular reflector in behind. That produces a nice pattern of energy, nice in that it's mostly symmetrical.

If you take a light that uses two flat reflectors, like the UVCAN Gerani, isotropic goes away. UVCAN Gerani far uv light fixture
The same polar plot of the two 'cuts' of incident energy is now very different. In the blue cut, those flat reflectors now produce a pattern with two distinct lobes. The graph is normalized energy, normalized against the peak measured.

What's going on here? What does it mean? A polar plot of the Gerani far uv incident energy in two cuts 90 degrees apart
The reflectors create a unique pattern that concentrates energy in the two blue lobes, but only in the cut across the middle. The other pattern is a broader kindof isotropic. If you can imagine that in 3D, this would form a very interesting shape.
Now recall the Delphi and my dining room application. If you're sitting 30 degrees off centre, the power in the pattern is quite low - under half the peak.

If I used the Gerani in that application, now the peak power is pointed exactly along the 30 degree lines...right at guests
Performing an exposure energy calculation in this case is much more challenging, and past the ends of the filter window I'll have to interpolate between the two patterns in 3D. I likely can't use this in that ceiling mount because of the lobes, and I'm still working on the model.
You need to know the beam pattern and beam width of any uv source that is of any larger power. This is because of the potential of exceeding recommended exposure limits for eyes and skin.

A low power light (Nukit 3W) that comes with a distance recommendation (1 m) is no problem.
Isotropic approximations can be very useful, but only in the case that a light does not have a lobed pattern. This is the Lily, 3W, but can see it's reflector design is also producing lobes. Polar plot of energy emitted by the UV CAN Lily far UV light
On the Nukit, kind public domain measurement by @EwanEadie is available in tabular form. Since the Nukit has a semi-circular reflector, my guess is it has a nice pattern in both cuts like the Delphi Industrial.

The data are here for the Nukit. These are super lights imo, very flexible. I've used these lots. Thanks to @NukitToBeSure for your commitment to quality and cost-effective design! (just received a note for a US brand of uv @ $2,000 'on sale').
researchgate.net/publication/37…
This is plotted cartesian not polar, but the same data. Can use that isotropic approx within the main beam, which is about +/- 33 degrees beamwidth to the half power points (50%).

I've changed the scale a bit. A Cartesian plot of the Nukit light normalized energy in only one axis.
If the Nukit had two flat reflectors, would expect to see a strongly lobed pattern.

Designs with flat reflectors may be looking to concentrate energy. More likely it's chosen for it's lower cost.

For most applications a symmetrical energy pattern is preferred.
So, tldr, be aware of what the light design is putting out in terms of an energy pattern, to determine if you can use simplified exposure math, or if you need to break out a spherical coordinate system (or ask an expert).
This is not engineering advice. If you're going to use higher power uv at any frequency, consult appropriate knowledgeable people concerning safe exposure limits. This is particularly true with very high power (100 watt+) fixtures.

Not a light expert, but know ++ about antennas
I have a 20W Gerani fixture and the Nukit set.

Use the 20W in an upper air application where those two lobes are pointed along the ceiling. Not a bad location for that pattern.

Use Nukit on table top regularly. Being able to use 4x 3W resolves much of the dose concern. /fin
PS. If you’re trying to do irradiance measurements, you need to have some idea if the light is pushing out a symmetrical pattern (balloon) or not (lobes). If it’s lobes, you can’t do a simple measurement on centreline with distance.

For the Gerani you’d measure half peak. Image

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More from @sameo416

Jan 25
I’ve been very vocal about bishops who have no courage to undertake real change. I relinquished my license as a priest after 18 years to protest failures to seek reconciliation with Indigenous and to care for survivors or clergy sexual abuse.

It’s good to see this courage.
is the clergy sexual abuse history.

There’s one other stand out recently, the Bishop of Newcastle, Rt Reverend Helen-Ann Hartley. Her courage directly led to the resignation of the Archbishop of Canterbury over a failure to intervene in clergy misconduct.acctoo.ca
The similar patterns repeat, consistent with what I’ve observed in engineering failure response.

The first priority is to get everyone to fall in line and to protect the institution and the decision makers who screwed up.

If a victim is needed a mid level manager is given.
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I wonder if this time around it will start to click for people.

If you live life not giving a damn for those around you, who you vote for is mostly irrelevant.

It’s a broken system, not a broken political party.

And the way you fix it is by starting to give a damn about others
At a panel discussion talking about transforming society to support effective climate advocacy.

I talked about relationally and responsibility in Métis cosmologies.

After a couple of hours of good presentation, one person says - this is supposed to be about climate action.
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(Asker an engineer, got that ‘fix the problem’ bias loud and clear)

Me (having just explained the transformation needed):
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Excellent thread as always.

One of the things I’m getting tired of saying - none of this should be a surprise. Colonial systems are built on violence and killing/silencing is just another tool in the bag.

Indigenous People’s know this all too well.
We’ve lived in a society that actively seeks our death or elimination so it can continue unbridled consumption and extraction.

The difference now is that violence is turned on everyone who resists the ‘just a cold’ ‘get back to the office’ ‘dinner out to support the economy!’.
What I’ve said over and over - none of this should be a surprise.

Remember those Indigenous voices saying that colonial societies are intrinsically violent?

This is what it’s like to live under violence.

And the deep grief that goes with reinforced trauma for generations.
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In the UK Covid Inquiry, some ? comments by infection control and public health voices (Ritchie and Hopkins) about respirators/'scientific evidence'. Hopkins says here, 'evidence is weak that FFP3s protected more than fluid resistant surgical masks (FRSM)'
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The distinction has validity but not when used to degrade evidence.
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A word of explanation if exceptionalism and colonialism draw a puzzled look.

Exceptionalism, at least in my engineer’s definition, relates to a default assumption that your reality is/should be universal.

Someone who doesn’t conform to your reality is marked as dim/dumb.
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I was in a line in Heidelberg, man behind me says “so nice to hear another American after nothing but German”. Polite exceptionalism.
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The assumption someone speaking English is American is an aspect of that exceptionalism.

It also exists in the belief that everyone should understand your culture.
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A very balanced response to a commentary that uses pejorative descriptors to distance those of us who have repeatedly called out the systematic failures of PH.

My advocacy is neither ‘obsessive’ nor ‘fixated’ as the op characterized it.
It is very difficult to take someone seriously when they use those sorts of descriptors to characterize a position they are critiquing.

They’re broad and undefined and hand waves intended to disparage the opposing position.

We’ve detailed many examples of failures to think.
In PH and infection control. Specific examples with receipts.

And infection control is still promoting droplet precautions for Covid. This is not amusing, it’s negligent and dogmatic.

Without deep changes to medicine and PH, we will repeat the same errors every single time.
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