Light – Pupil lumens and Design lumens
Historically, light has been measured using ‘lumens’, as measured by conventional lux meters. However, current thinking is that the lux levels recorded by a lux meter, tell only part of the story. It is now fairly common to see light output described in ‘initial design lumens’ and restated as ‘initial pupil lumens’.
The theory behind this is that the human eye ‘sees’ through light receptors called rods and cones. The cones respond to light in the yellow / green spectrum and the rods respond to light in the blue/ green spectrum.
There are roughly ten times as many rods as there are cones in the human eye – but a conventional lux meter predominantly measures the light that the cones would respond to.
That means that the lux meter is ignoring the majority of the light that you actually need to see clearly.
Two scientists – Dr Sam Berman and Dr Don Jewett have developed a new measurement system – their work was supported by the US Energy Department – not by a lighting company trying to argue the case for LED lighting. Their research is the basis for the conversion ratio (P/S) that is used to convert ‘initial design lumens’ to ‘initial pupil lumens’.
The theory behind this is that the human eye ‘sees’ through light receptors called rods and cones. The cones respond to light in the yellow / green spectrum and the rods respond to light in the blue/ green spectrum.
There are roughly ten times as many rods as there are cones in the human eye – but a conventional lux meter predominantly measures the light that the cones would respond to.
That means that the lux meter is ignoring the majority of the light that you actually need to see clearly.
Two scientists – Dr Sam Berman and Dr Don Jewett have developed a new measurement system – their work was supported by the US Energy Department – not by a lighting company trying to argue the case for LED lighting. Their research is the basis for the conversion ratio (P/S) that is used to convert ‘initial design lumens’ to ‘initial pupil lumens’.
Under the ‘Pupil lumen’ measurement system, Induction lighting performs very well – much better than HPS for example. As do Metal Halide, T5, T8 and LED light sources.
When considering what type of lighting you need, there are intangible factors as well as the hard facts to consider. One of the ‘intangibles’ is the perceived brightness or ‘punch’ of the light source.
Induction lamps produce much of their light in the green/blue spectrum – a HPS lamp is weighted towards the yellow/orange end of the light spectrum. As a result, the lumen output of an Induction lamp is increased to convert from design lumens to pupil lumens, whilst that of a HPS, is significantly reduced.
But, the HPS unit is producing a lot of light energy – it’s just that a lot of the output doesn’t help you see any better – it just produces ‘glare’. As a result, the HPS source can appear much brighter, but actually produces less ‘visibility’.
For very high-level lighting – in our view above 30 feet – the benefits of Induction lighting start to fade, along with the light output, as distance takes its toll. Induction lamps are great for broad area illumination – they are not point source lamps.
If you would like to read more about this – we suggest you download the street lighting research study, carried out in the USA in January 2008.
When considering what type of lighting you need, there are intangible factors as well as the hard facts to consider. One of the ‘intangibles’ is the perceived brightness or ‘punch’ of the light source.
Induction lamps produce much of their light in the green/blue spectrum – a HPS lamp is weighted towards the yellow/orange end of the light spectrum. As a result, the lumen output of an Induction lamp is increased to convert from design lumens to pupil lumens, whilst that of a HPS, is significantly reduced.
But, the HPS unit is producing a lot of light energy – it’s just that a lot of the output doesn’t help you see any better – it just produces ‘glare’. As a result, the HPS source can appear much brighter, but actually produces less ‘visibility’.
For very high-level lighting – in our view above 30 feet – the benefits of Induction lighting start to fade, along with the light output, as distance takes its toll. Induction lamps are great for broad area illumination – they are not point source lamps.
If you would like to read more about this – we suggest you download the street lighting research study, carried out in the USA in January 2008.
Street Lighting Research Study (937 KB)
Pupil Lumen and design lumen output
For most outdoor applications, pupil lumens can be utilized to recommend 1:1 replacements. For example you could replace a 150W HPS system with a 80W induction system.
The following table will help you compare the ‘design and pupil lumen’ outputs of the various types of lamp available and to select the Induction lamp equivalent.
- Traditional light measuring meters have been calibrated for use in the photo industry (thus the term Photopic meter). Most designers and architects will use photopic readings (design lumens) to build and recommend lighting solutions.
- Unfortunately, photopic meters can be deceptive in that what is bright to the machine is not always what is bright.
- An example of this is the yellow light that is emitted from Low Pressure Sodium, which has an extremely high design lumen readings, but low pupil lumen readings.
- For most outdoor applications, pupil lumens can be utilized to recommend 1:1 replacements. For example you could replace a 150W HPS system with a 80W induction system.
As well as the initial lumen output, the light output depreciation should also be taken into consideration when choosing the best type of light source for a particular application.
One of the key benefits of an Induction lamp is the stability of the lumen output over time. At 90,000 hours, the lamp should still be producing around 80% of its initial light output.
That compares incredibly well with metal halide – which would have been on the sixth replacement by that time, or HPS, which would have been replaced five times in the same period.
That compares incredibly well with metal halide – which would have been on the sixth replacement by that time, or HPS, which would have been replaced five times in the same period.
Lifetime – reliability
It is a fact that an Induction lamp can last 100,000 hours. It is not true to claim that all will, or do.
As with everything in life there are good ones and ones that are best avoided.
100,000 hours is a very long time – running 24/7 every day – we would need to wait nearly 11.5 years to make sure the lamp had reached its claimed life.
One of the worlds leading Induction lighting companies is US LightingTech, based in Irvine California. They are an original equipment design and manufacturing company working closely with Sylvania Osram and GE. We are now their European partner and unashamedly have gained a huge amount of knowledge and research from them as a result.
As a result, we know which products will do what they claim and which ones don’t. We know the main causes of premature failure and the traps to avoid. Importantly we have been briefed on a mortality study that showed that at 100,000 hours less than 2% of the lamps used had failed in use.
As a result we offer a full five-year warranty on our kits and a ten - year warranty on our OE lighting units.
The chart below illustrates the number of relamps over the course of 100,000 hours.
