Green or Red Laser Diode Module: How to choose?Follow article
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Choosing between a green or red wavelength in your laser alignment, positioning, or targeting application is a common cause for concern. However, by evaluating the ambient lighting conditions in which your application takes place, the projection and observation distances involved, the laser output power/safety requirement, and by knowing the means of detection, you’ll find that it is a relatively straightforward decision. This article will guide you through that decision-making process, where Table 1 provides a very brief overview. The remaining content provides context surrounding Table 1.
Table1. An overview of the decision-making process behind a red or green laser diode module for your alignment, positioning or targeting application. Hyperlinks will take you to a relevant laser diode module product.
High ambient light (daylight or well-lit room)
Low ambient light (nighttime or dark room)2
1. 650/660 nm is preferred in this instance.
2. Consider projection/observation distance and output power/safety requirement.
3. 635 nm is preferred to 650/660 nm in this instance.
Green and red wavelengths are a part of the visible portion of the electromagnetic spectrum, spanning roughly 400-700 nm. Typical green laser wavelengths emitted by semiconductor laser technologies include 515, 520, and 532 nm. Laser diodes emitting red laser wavelengths are more abundant and include, but are not limited to, 635, 640, 650, 660, 670, 685, and 690 nm.
The most used wavelengths in these colours are arguably 515, 520, 532, 635, 650, and 660 nm. The reason for this is typically economical, as laser diodes emitting these wavelengths are usually lower cost and/or easier to obtain in small or large quantities. The remaining wavelengths are considered to be more specialised, although the cost difference between them and the more common variants is sometimes marginal.
If you do not have a specific wavelength requirement for a specialised application and simply want to choose between green and red then, as a rule of thumb, choose between 515, 520, 635, 650, and 660 nm. The choice is made even easier when you know what laser power or safety requirement you need, as laser diodes emitting 515 nm transition to 520 nm at approximately >10 mW. Similarly, laser diodes emitting 650 nm transition to 660 nm at approximately the same output power. Thus, your choice will be narrowed to 515/520 nm, 635 nm, and 650/660 nm depending on your required output power or your laser safety requirement.
The ambient lighting conditions in which your application takes place will influence your decision. For example, if your application takes place outdoors and in daylight hours, then there will be an enormous amount of ambient light for the laser beam to contrast with. If the alignment, positioning, or targeting task is to be completed by the eye in this instance, then a laser diode emitting 515/520 nm will be beneficial. According to the CIE photopic luminosity function (see Figure 1), this is because the human eye is approximately seven times and three and a half times more sensitive to 520 nm (green) light than the equivalent power in 660 nm and 635 nm (red) light, respectively.
Your choice of wavelength also depends on whether the alignment, positioning or targeting task will be performed by eye or through the aid of cameras. This is because the spectral sensitivity of the human eye and typical camera technologies are distinctly different (see Figure 1). The sensitivity of the eye peaks at around 550 nm in well-lit conditions (known as photopic vision). The peak shifts to 507 nm in low light and very dark conditions (mesopic and scotopic vision). In contrast, typical camera technologies such as CMOS and CCD exhibit peak sensitivity at larger wavelengths (>600 nm). As a result, in general, it is beneficial to use green light for tasks performed by eye and red light imaging/digital alignment applications.
Exceptions to this generality include scenarios where the projection and observation distances are particularly short (or, equivalently, where high output laser powers are used over larger distances) in applications performed by the eye, as the eye’s lens can collect a greater amount of red light to account for the reduced sensitivity. In these examples, the red light of choice is 635 nm rather than 650/660 nm since the eye is more sensitive to this wavelength.
If your application is to be performed by a combination of eye and camera, then it is simply a case weighing the pros and cons of green and red light and deciding whether visibility or camera sensitivity is more important. It is wise to perform tests to ensure the application can be completed successfully.
Figure 1. Typical spectral sensitivity curves for CCD and CMOS cameras and the human eye (photopic vision).
Projection and observation distances
The projection and observation distances involved in your application will also affect your choice of wavelength. If the projection (working) distance is large, then there will be a greater loss of laser power through scattering and attenuation. This means that less power reaches the target surface compared to shorter working distances. Equivalently, if the observation distance is large, then less power reaches the observer. These factors can make illumination more difficult to detect. In these instances, it is beneficial to select a wavelength that generates a greater response in the detection scheme. Conversely, if the distances involved are very short then the wavelength becomes less important.
Output power/laser safety requirement
Two of the main parameters when choosing a laser diode module is the wavelength and output power. The output power you choose is often determined by your laser safety requirement. A laser safety officer (LSO) internal to your organisation will help you make the correct decision. Given thorough details of your application, some laser suppliers can also make suggestions on what output power you will need to achieve a certain laser safety classification. If the output power is limited by a stringent safety requirement, then the illumination might be difficult to see or detect. Your choice of wavelength then depends heavily on the spectral sensitivity of the detection scheme (eye or camera).
In some applications, laser safety is less important and beam detection is a priority (laser safety glasses are highly recommended in this instance; your internal LSO or laser supplier can help you choose the correct eyewear). In this case, the higher output power will often be implemented to maximise the response in the detection scheme, and laser wavelength becomes less important.