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Hi Phil and others,

Answering this question can be a bit complex as there is so much variation in 
power across the Hg spectrum. There is also some variation across the 
LED/solid state hybrid spectrum through it is not so extreme. 

In general Hg wins on the sharp peaks but for some time LEDs have won in the 
Hg troughs, the main one being between 435nm and 546nm. In most cases 
users should now be getting similar results to a new Hg across the spectrum 
and more than adequate power for most general wide-field fluorescence work. 

If we consider the spectrum split up into 4 bands, violet, blue, green-yellow 
and red-NIR we can explain comparisons in a bit more detail. LEDs in the blue 
from 440-500nm have been stronger than the Hg for quite some time as in the 
red-NIR region. Both these regions are in Hg troughs. The metal halide bulbs 
have done a good job filling up the blue trough and its maybe only over the last 
couple of years that LEDs are now beating the metal halide in this region. The 
violet region which I would put between 365-440nm has been adequate for 
some time, most people commenting that DAPI is always too bright. There has 
been a lot of progress on power over this region with 365nm now being more 
than good enough in most cases. The green-yellow region has been the most 
problematic area. Lumencor were the first to deliver good power in this region 
and stood out for some time with the phosphor rod technology they use to 
cover this region. Other LED technologies have caught up on this now, so 
bright green should now be expected from other vendors. At CoolLED we have 
seen our green-yellow region improve by as much as 6 times over the past 
year. 
  
This is all great news however there still can be disappointment from LED 
systems and I believe this is down to the limited spectrum. This means that if 
filters and LED peaks do not match up then results will be poor. With bulbs 
users did not need to be so conscious of filter sets but with LEDs it is crucial. 
For example a user may purchase a ‘broad spectrum’ LED/solid state light 
source to find that no DAPI is visible as no 365nm is available. In such cases a 
new filter set is needed. You may also specify an LED system with 460nm or 
470nm that is great for GFP with most cubes. This can fall down however when 
a multiband filter set is used which pushes the GFP excitation passband up to 
around 490nm. Ideally one would like both the 470nm and 490nm included in 
the LED system. Addressing this problem has been a major issue for LED system 
manufacturers. Some get around this by offering swappable LED modules but 
this brings other issues like swapping time and ensuring combining dichroics are 
correct. Just adding more wavelengths is another option but this has a big 
overhead with LED systems as each new source requires its own drive 
electronics, collimating optics and combining dichroics. Adding more 
wavelengths in the conventional way has an increasingly negative impact on 
overall power mainly due to the dichroic losses. This has limited the number of 
peaks in a system typically to around 4 but up to 6 or 7 at most. 

The good news is we have now addressed this limitation with a novel approach 
at CoolLED and will be showing our 16 selectable wavelength system, 365-
770nm, at Neuroscience next week. 

I’m happy to explain more on this to anyone that is interested. 

Kind regards,

Gerry

Technical Manager
CoolLED