The use of cameras, filters and lights in art conservation is hard to overstate. Every object that is warmer than absolute zero emits energy in certain wavelengths. Objects will also reflect and absorb energy at certain wavelength and emit energy at another wavelength which can also be thought of as a type of fluorescence.
When you understand how an object Reflects, Absorbs, Transmits, Emits and Fluoresces (RATEF properties), you can learn a lot about the object.
You might be trying to detect skin cancer, detect if the olive oil being sold is actually corn oil, detect if a diamond is really a diamond, prospect for minerals, see were crops need to be fertilized, estimate biomass, detect if a painting has been altered, see under drawings and on and on. There really is that there is almost no limit to what you can learn when you understand the concept - and know what look for having the right equipment.
In art conservation, there are a few standard ways that art is analyzed. Typically, a digital camera modified for full spectrum (a UV-VIS-IR conversion) is used various lights and camera filters. Some examples:
- UV Absorption / Reflection. While a color UV-VIS-IR camera can do this, a monochrome UV-VIS-IR camera has about 6x more UV sensitivity. Camera is looking for where an object absorbs light in the UV but not in the visible or infrared ranges. For instance, suntan lotion contains UV absorbers that are usually clear in the visible and IR. Skin bruises can be more easily seen in the UV. A painting that has been touched up may look different in the UV. This requires a camera that can see in the UV and a UV light source. Additional UV short pass filters may be necessary depending on the setup.
- UV to Visible Fluorescence. A standard digital camera can be used for this since the camera is capturing a visible light emission. A UV excitation source (light or flash) is used. With a UV-VIS-IR camera, you need a UV blocking filter. You may also need an IR blocking filter like our XNiteBP1 filter on a UV-VIS-IR camera because any slight IR leakage will contaminate the picture.
- UV to IR Fluorescence. A UV-VIS-IR camera with an IR longpass filter on the lens. Excitation source is a UV flash. Some pigments will emit in the infrared when excited with ultraviolet light. This can be a very useful technique to recover lost writings such as water damaged, burnt and faded letters.
- Visible Bandpass. A standard camera can be used with either various bandpass filters on the camera or by restricting the light in the room and illuminating with specific light sources. We have a wide range of XNiteFlashF series flashlights that emit light from the short UV to near IR. If the flashlight is the only source of illumination in the room, then you don't need to use and filter on the camera. This is also known as Poor Man's Multispectral Imaging.
- Visible to IR fluorescence. Certain pigments can be excited in the visible and will fluoresce in the infrared. Egyptian Blue is an example. Trace amounts of Egyptian Blue that can't be seen by the human eye can be picked up by illuminating the object with a strong visible light that doesn't have much IR emission and using a IR longpass filter on a UV-VIS-IR camera.
- IR Absorption, Bandpass, Translucence. Certain pigments and objects become translucent in the IR allowing you to see through them. Common examples are looking for charcoal underdrawings on paintings.
Most objects RATEF properties are that the object goes from high energy shorter wavelengths to lower energy longer wavelengths. For instance, with a UV fluorescent object, you can excite the object with a higher energy UV light (UV light absorption) and visible light is emitted. The visible light has a longer, lower energy wavelength. Similarly, an object may block a short higher energy wavelength but pass a lower energy wavelength - for example suntan lotion blocking UV light but pass visible and IR light. Or a painting pigment might become translucent in the upper infrared while absorbing and reflecting light in shorter wavelengths. Usually, it doesn't work the other way around though there are occasional examples of an object going from lower energy to higher energy. An example is an up-conversion phosphor. Such phosphors can be found in many secure documents as a form of authentication. The up-conversion phosphors works by using multiple low energy photons to progressively bump of the energy level of an electron until it has sufficient energy to collapse back to its normal state and emit a higher energy photon.