Source Lumens vs. Delivered Lumens
When selecting a luminaire for use in a hazardous area, there are lots of different factors to consider. The lumen output (or power) of the luminaire, is one of these important considerations. But are all lumen output figures comparable? In this article, we explore the difference between ‘source lumens’ and ‘delivered lumens’.
Source lumens: The ‘source lumens’ value is the total amount of light the luminaire emits at source, before it is transmitted through any lens or diffuser. Lumens are lost with every reflection, refraction or absorption through each material the light passes through resulting in the value of ‘delivered lumens’ being less than the stated, emitted figure. The output can also be blocked or dispersed due to the luminaire’s mechanical build interrupting the light flow, particularly at the extremities of the beam angle.
Delivered lumens: ‘Delivered lumens’ is the most accurate measure of how the luminaire will perform once installed. It is the actual amount of light delivered on scene, taking into account all barriers such as lenses, housing, and any filtering effects.
Now we know that lumens can be measured in two different ways and will produce two very different figures. Now we’ll investigate the impact of this in more detail.
Which Figure is Used by Hazardous Area Lighting Manufacturers?
There is a large degree of uncertainty when comparing lumen output figures from different manufacturers. The ‘delivered’ value is considered the more equitable figure to use, but with no official standard to follow, there is nothing to say that this is a value which all manufacturers are using. Identifying which figure is being used can often be very unclear and difficult to identify. This is especially true if photometric files (which give a true reflection of performance) are not made readily available.
Imagine the scenario, you’re specifying lighting for a project and are choosing between two luminaires from different manufacturers. Unbeknown to you, each manufacturer has calculated the lumen output value in different ways; luminaire A is using ‘source lumens’ and luminaire B is using ‘delivered lumens’. On paper, the performance of two luminaires may seem very similar, but the reality is likely to be very different.
25K Lumens at Source:
25K Delivered Lumens:
As a customer, do you query the ‘delivered lumens’ figure when purchasing a luminaire?
Importance of Lighting Design
One way to overcome these issues, and to get greater clarity on a luminaire’s performance, is through lighting design.
To create photometric files (such as IES or LDT files) for professional lighting designs, luminaires need to have undergone photometric testing, usually done by an independent third party. Photometric testing measures the colour, quality and exact lumens that the luminaires deliver on scene, not the total lumens at source. This is where lighting design becomes an important factor. Lighting design provides you with a visual representation of the final appearance of the installed lighting solution, demonstrating the lux levels and spread of light at the point of application. This provides a true indication of lighting performance before purchase, removing any ambiguity from the lumen figure.
Delivered lux levels can then be compared back against the lighting design report to ensure you’re achieving the light levels expected.
The Raytec Approach
At Raytec, we always display our lumen output figure as ‘delivered lumens’. This means our figures are completely transparent, so you’ll achieve exactly the performance you’d expect from the figures we publish. Going forward, we’ll also be updating our literature to include both ‘total lumens’ and ‘delivered lumens’ for even greater clarity.
Photometrics files for all our products can also be download directly from our website, and we also offer a free of charge lighting design service where our team of experts can help you design a custom scheme to ensure excellent light output and performance.
Summary
The lens of a LED luminaire has a very significant role in the way the light is spread and transmitted to the target source. Consideration often isn’t given to the light transmission of the inserted lens or visors. Even a high-quality, clear glass lens that appears to the human eye to be completely transparent can result in a light loss which goes unstated by many manufacturers. The effect of this could have a significant impact on the perceived brightness of the final installation.
If it’s not clear, query whether the lumen output figure is ‘delivered’ or ‘at source’ and use lighting design where possible to get a true reflection of performance.
Designing a luminaire with a high total lumen figure is relatively easy, but the real engineering challenge comes from the overall design and ensuring as much as possible of this total figure is actually being ‘delivered’. Furthermore, this needs to be achieved while keeping the LEDs with proper thermal management to ensure long life and reliability.
To learn more about Ex lighting, call us on +44 (0) 1670 520 055, or email Raytec Global at sales@raytecled.com or Raytec Americas at ussales@raytecled.com.
