A Longpass Dichroic Mirror (also known as a longpass dichroic beamsplitter) is a specialized optical filter used to separate light based on wavelength. Its primary function is to reflect shorter wavelengths while allowing longer wavelengths to pass through (transmit).
These components are essential in fluorescence microscopy, multi-laser systems, and spectral analysis, acting as the primary "traffic controller" for directing different colors of light.
Working Principle
The term "Dichroic" implies interaction with two distinct color bands, while "Longpass" describes the transmission behavior: it allows "long" wavelengths to pass.
- Orientation: The mirror is typically oriented at a 45° Angle of Incidence (AOI) relative to the incoming light source.
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Mechanism:
- Reflection (Short Wavelengths): Light with a wavelength below the cutoff point hits the mirror surface and is reflected 90°.
- Transmission (Long Wavelengths): Light with a wavelength above the cutoff point passes straight through the glass substrate.
Key Terminology
- Cut-on Wavelength: The specific wavelength where the filter transitions from reflecting to transmitting light. Technically, this is defined as the point where transmission reaches 50% of its peak.
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- Context: A "500 nm Longpass" will reflect light below 500 nm and transmit light above 500 nm.
- Reflection Band (Stopband): The spectral region where the component reflects incident light. In a longpass dichroic, this consists of the shorter wavelengths (below the cut-on). High reflectivity (>98%) is desired here to preserve signal intensity.
- Transmission Band (Passband): The spectral region where the component transmits incident light. In a longpass dichroic, this consists of the longer wavelengths (above the cut-on).
- Transition Width (Slope): The sharpness of the change between the reflection band and the transmission band. A "steep" slope (e.g., <5 nm) is critical for separating wavelengths that are very close together.
Spectral Characteristics
On a transmission graph (Transmission % vs. Wavelength), a Longpass Dichroic Mirror resembles a "step" function that steps up.
- The "Step Up": The graph starts at ≈ 0% transmission (reflection zone) for UV/Blue wavelengths and sharply rises to >90% transmission for Green/Red/IR wavelengths.
Example Application: Fluorescence Microscopy (DMLP505)
A standard industry example is the 505 nm Longpass Dichroic (DMLP505), widely used for visualizing Green Fluorescent Protein (GFP).
- Cut-on Wavelength: 505 nm
- Reflection Band: 380 nm - 495 nm (Blue/UV)
- Transmission Band: 515 nm - 700+ nm (Green/Red)
How it works in practice:
- Excitation: A blue light source (470 nm) hits the mirror. Since 470 nm < 505 nm, the mirror reflects the blue light down to the sample to excite the GFP.
- Emission: The sample glows, emitting green light (525 nm). This light travels back up to the mirror. Since 525 nm > 505 nm, the mirror transmits the green light straight through to the camera.
