Manufacturers produce either silver- or copper-coated prisms to reflect laser wavelengths, but what's the difference?
In the early days of electronic distance measurement (EDM), in the mid to late-20th century, there was a school of thought that copper-coated prisms better reflected the laser wavelength of the instrument and not as much of the visual wavelengths. This may have been true back then when EDM instruments had very weak light sources when compared to today's standards. Today's EDMs have much stronger lasers and narrower bandpass filters on the receiver, making any difference negligible.
The simple answer is that a lot of it comes down to history, and in the case of Trimble®, silver-coated prisms have been the standard for decades. Let's explore why.
The graph below shows the reflectivity of different metals as far as Trimble instruments are concerned, where the gold line (Ag) is silver and the red line (Cu) is copper. As you can see, silver and copper practically have the same reflectivity.
It's important to note that when referring to the metal coating of a prism, it is only the coating on the back surface of the prism, which gives the high reflectivity for the distance meter. A coating can also be applied to the front surface of the prism to avoid any unwanted additional reflection (more on this later).
Trimble's EDM systems work at the wavelength of 0.66, 0.77 - 0.92 and at 1.55 µm, which means that the base reflectivity for silver and copper is at the same value. However, silver-coated prisms look brighter and white when aiming, while copper-coated prisms look orange.
The coating on the front surface of the prism, if applied, is typically an anti-reflective coating best suited to the wavelength used in the instrument. Both Leica and Trimble have used this technique for many years on some prisms.
One of the drawbacks of using the anti-reflective coating technique is that the prism is then tied to specific EDM wavelengths because the coating lowers the reflectivity for some wavelengths by increasing it for others.
Trimble added an anti-reflective coating to our traverse prisms after the introduction of the SX10 scanning total station, since the behavior of that EDM laser created some minor inaccuracies due to the reflex in the surface of the prism; that coating is optimized for both the SX and S Series instruments (as well as any future planned instruments).
For Trimble optical users, issues can occur if copper-coated prisms with anti-reflection coating are used with an SX Series instrument. While there is no issue with the copper itself, the anti-reflection coating on the front of the prism is optimized for Leica EDMs which use a wavelength at 0.85µm and 0.66µm, as opposed to the wavelength of the Trimble SX Series EDM, which is at 1.55µm, thus the front reflection is larger.
The reflection in the front surface of the prism can give a pointing error (up to an additional 1.5mm or 0.005ft), or in worst cases, the measurement is refused, especially if the prism is tilted.
Trimble-branded silver-coated prisms have well specified and controlled surface flatness. Cheap prism copies using a copper coating or silver-coated prism may also have poor range with Trimble systems due to poor surface flatness.
There's more to prisms coating than meets the eye, and using genuine prisms purchased from your total station manufacturer will always achieve the best results and the best deliverables.