Interference. High THD can cause electromagnetic interference (EMI) and other disruptions in nearby electronic devices. Lower THD reduces the risk of interference and makes the LED fixtures more compatible with other equipment.

Opticaldistortionmeaning

Power Quality. High THD can disrupt power quality for other equipment in the facility. As power quality of the facility decreases, the electrical system will have less stable voltage and current, which affects the overall performance of the electrical system and other devices.

Distortion lightapp

The best way to determine the resolution of an optical system is to test it directly using MTF testing. One can purchase expensive MTF testers or pay someone to do the testing, but we like cost effective methods. Read below to learn more about MTF testing and an inexpensive testing method we’ve developed.

LEDs themselves don’t generate significant THD. However, the power supplies (drivers) that convert AC power to DC power for the LEDs can. Low-quality drivers might introduce higher THD. So, it’s important to understand the LED system in its entirety.

Distortionaberration

Pincushiondistortion

Imagine a perfect electrical current waveform as a smooth sine wave. Electrical devices often introduce “harmonics,” additional current components at multiples of the main frequency (usually 60 Hz in the US). These harmonics distort the original sine wave, creating total harmonic distortion. Total harmonic distortion is expressed as a percentage. In this post, we’ll explain why lower percentages are beneficial when considering an LED conversion in your facility.

LensdistortionExamples

The switch to LED lighting offers tremendous benefits – energy savings, longer lifespans, and improved light quality. However, there’s one technical aspect to consider: Total Harmonic Distortion (THD).

Reliability. Lower THD can improve the reliability of the LED fixtures and other devices on the same electrical system. This can reduce maintenance costs and increase the overall lifespan of the equipment.

Barreldistortion

Heat Generation. Higher harmonic distortion means the system is working harder to produce what it is expected to produce. This increases heat generation in the electrical wiring and components, increasing the risk of overheating and shortening the lifespan of the fixture and other connected devices.

The next problem is how to do this cost effectively. Have you heard about the Raspberry Pi educational computer? You can purchase one for $35 and add a camera for another $30. Even with the SD card, wireless network adaptor and so on, your investment should be under $100. Unless you already have the parts handy, it will probably cost more for an optical rail and the parts to hold the lens, camera, and point source than it does for the camera to do the job. The Raspberry Pi camera has 1.4 micron pixels, so it has enough resolution to test up to 180 cy/mm, which is plenty for most lenses. All you have to do is remove the lens from the camera (just as you would do for the resolution chart tester) and figure out how to hold the camera and Pi. We built a custom holder with an M4 thread for mounting to a post. Here is a picture of our setup:

The simplest possible point source is a laser diode module. You can buy an inexpensive one with a driver circuit on Ebay for less than $10. High power is undesirable and may damage the camera; 5mW is plenty. I power my laser diode with a lab power supply, but you can run the module off of batteries. One thing to verify before you buy is whether you can remove the collimating lens from the module – you’ll need to do this for a point source. If your lens needs to be tested with collimated light, you’re better off buying a high quality collimating lens from a supplier like Edmund Optical, Thorlabs, or Newport. Just make sure the lens diameter is larger than the entrance pupil of the lens you’re testing. One problem with lasers is that they emit coherent light, which causes problems if you want to measure incoherent (normal) MTF. The solution to this is to limit the current to the laser diode so it is below threshold and emits spontaneous (incoherent) light rather than stimulated (coherent) light.

Energy Efficiency. Some may think that as long as you’re switching out higher consumption lighting for an LED, you’ll be efficient. Well, distorted current can lead to higher energy consumption and negate some of the energy-saving benefits of LEDs. It’s important to have LED fixtures with lower THD so the electrical current is closer to a pure sinusoidal wave, which improves energy efficiency.

Distortion lightcar

Let’s start with the theory behind MTF testing. The basic idea is that the MTF of an optical system is a slice through the Fourier transform of the point spread function. Too much jargon? OK, let’s break it down. If you have not yet learned about MTF, please read our page on MTF and then come back. Next, a Fourier transform is a somewhat complicated mathematical operation. When performed by a computer, it is typically called an FFT (fast Fourier transform). Finally, the point spread function is just the image of a point source (like a star) formed by the lens. So all we have to do to measure MTF is take a high resolution picture of the image of a point source, take a cross-section of it (horizontal or vertical to make it easy), and have a computer do an FFT.

It’s important to consider THD in your LED upgrade, especially in manufacturing plants where processes can be easily affected by power quality and harmonics. Many manufacturers only advertise the life of their LED chips but do not consider the system as a whole. By understanding THD and making informed choices, you can ensure your LED lighting upgrade delivers optimal benefits without compromising the performance and efficiency of your entire electrical system.

If you want to measure lenses with high MTF, a bare laser diode module might not be enough. They tend to be expensive, but a spatial filter will refocus the laser through a pinhole and assure you of a high quality point source. If you want to know what size pinhole to use for the spatial filter, the diameter should be

As always, we’re interested in finding creative ways to solve complicated (or expensive) problems. If we can help you with your next project, let us know!