Reflected light

Reflected light is used when a specimen is opaque. It's obvious that we can't shove light through a thick metal specimen so how do we look at one? Well the answer is to bring the illuminating light down through the objective and use the objective as a condenser.

Since 99% of reflected light modules are used in materials quality control and research there will be a shifting of gears in this book. Specimen is now something used in industry; ie: metals, semi-conductors, materials of all kinds. Its to bad that biology hasn't discovered reflected light since it is really neat stuff.

For all you biology based users out there the industrial folks have developed ways of looking at large, opaque things that work just fine. They have long working distance objectives and all kinds of interesting stands. They also have heads that change angle to suit you. Take a look at industrial microscopes you bio folks, it will do you good.

The key to reflected light is the word reflected. We are dealing with reflection as well as refraction. If the material doesn't reflect then there is no image. Refraction is an image source from knife edged plateaus on the specimen. An example of this is semi-conductor lines located above the silicon substrate.

To get reflected light we must have a light source, a powerful one, and an illuminator. The light source should be a strong light source since many specimens in reflected light work are very dark, metals, coal, etc. Since the illuminator uses a half silvered mirror 50% of the illuminating light is lost at that point.

The illuminator is placed between the head and the top of the frame (in bench top microscopes) or the nosepiece (in research microscopes). The illuminator has a semi- silvered mirror that reflects the light from the light source down through the objective. The objective acts as both condenser and objective.

Most manufacturers now have illuminators, called universal illuminators, that let you use Nomarski, polarizing and fluorescence with dark field or bright field objectives. Even though a universal illuminator costs more it is worth it. With one you can do any technique with dark field objectives and just about any technique with bright field objectives.

The required mirrors and filters are contained in a unit called a "cube". These cubes can be purchased as needed and slipped into the illuminator. In some universal illuminators three or more cubes can be installed. Changing from one technique to another is as simple as pushing a lever.

Almost all bench top microscopes and all research microscopes can be equipped to do transmitted and reflected techniques simultaneously. This makes for a truly universal microscope, although most users would find its universality useless for their requirements.

Reflected Light Bright Field

Reflected light bright field is necessary for all the rest of the reflected light techniques. Bright field is used all by itself when ever the specimen has good contrast or a simple technique is needed. Simple bright field modules are small enough to mount on metal working machines and may have only one objective.

The basis of reflected light bright field is a good illuminator. Good illuminators have both field and condenser diaphragms. They act in a reflected illuminator as they do in a transmitted Kohler microscope.

To set up a reflected light microscope first close the field diaphragm. Some illuminators allow you to focus the image of the diaphragm but many are pre-focused. If you can focus the image do, but either way you need to center the image of the diaphragm in the center of the field. Now open the diaphragm until it fills the field.

Contrast is controlled by adjusting the iris (condenser) diaphragm. Be aware that if you use reflected dark field the iris diaphragm can really mess up dark field. Some manufacturers prevent this and others don't. Adjust the diaphragm for the contrast and depth of field you need.

Reflected light polarization

A reflected light polarizing module will include a polarizer located before the mirror and an analyser located after the mirror. While both may be rotatable to allow the most versatility usually only the analyser is rotatable. A well designed illuminator will allow the user to have either a very complex system with rotatable polarizer and analyser and to use wave plates or a very simple system with fixed polarizer and analyser. Wave plates and compensators should be easily interchangeable for a full range of polarizing techniques.

Obviously reflected light polarization requires a powerful light source. If you have a specimen that is dark then you need even more. The problem is that polarizers, being dark, absorb light and make heat. This can cook them, leaving them with holes and blotches. You must be careful and get a heat filter with any powerful light source. This will reduce the heat related failures in polarizers.

This technique is used throughout the materials area. A lot of things have birefringence and over the years we have learned to use this to identify materials. Anything with a crystalline structure may be a good candidate for this technique.

Reflected light fluorescence for materials

This technique works just like reflected light fluorescence for biological use however the specimen is usually a material like a semi-conductor wafer and the fluorochrome is something used in the manufacture of the material.

A good example of this is photo resist. This material is used in the production of semi-conductor chips. It must be thoroughly removed for the chip to work. It was discovered that this stuff fluoresced and soon fluorescent modules were being used in semi-conductor plants. Modern universal illuminators allow for rapid change over between fluorescence and all the other reflected light techniques.

Fluorescence is valuable only when there is a fluorochrome used in the material. Getting a fluorochrome into a material in a place that is of interest can be daunting to impossible. But were a fluorochrome exists and it is of interest this is an excellent technique.

The draw backs are the same with this technique as for biological fluorescence, light intensity. You need a lot in and you may not get a lot out. Typically a fluorescence equipped materials microscope will have a 75 watt or 150 watt Xenon light source. This light source provides dichroic illumination for bright field and other techniques and enough light for fluorescence.

These light sources may be way to bright for bright field and dark field. A universal illuminator should have a place for neutral density filters to reduce the intensity since these light sources can not be dimmed electronically. The best universal illuminators have built in neutral density filters that can be slipped in as needed.

A fluorescent equipped universal illuminator has three main filtering devices in it. The exciter filter takes the light from the light source and filters it to produce only the wavelengths needed to excite the specimen.

The next filtering device is both a filter and a mirror; a dichroic mirror. It reflects all light below a certain wavelength and transmits above that wavelength. This center point wave length must be between the excitation and emission point for the fluorochrome. It must be greater than the excitation wavelength so it will reflect the excitation light down and into the objective.

Dichroic mirrors act as filters by transmitting or reflecting unwanted light away. When the excitation light hits it any light that has to high a wavelength will be transmitted through the mirror and never reach the specimen.

At the specimen plane the excitation light triggers the fluorochrome to emit light. It will always be at a longer wave length than the excitation due to conservation of energy. The objective then gathers all the light that its NA allows it to and then the dichroic mirror passes the fluorescent light up to the barrier filter and reflects any excitation light back towards the light source.

The barrier filter screens out any left over exciter light and any fluorescent wavelengths that we don't want to see. The eyepieces then pass the image on to the user.

Obviously we must know the excitation and emission wavelength of the material we are working with. We must know these numbers to specify the filters we need to generate fluorescence. When we have this information then we can get the correct exciter, dichroic and barrier filter.

Most manufacturers put all these filters together in a unit, called a cube, that fits into the universal illuminator. There will also be cubes for bright field and dark field. Changing cubes is a simple as moving a lever. Manufacturers make cubes for all the most popular materials techniques such as photo resist. There are very good third party filter makers that can produce filters and dichroics in just about any wavelength and install them in a cube for your microscope.

What this means to you

Fluorescence is a valuable technique in materials science. You need to know the excitation and emission of the fluorochrome you are looking for. Lots of light is a must. Manufacturers have the tools ready for this technique.

Reflected light contrast enhancement

Its common in materials science that the material you are looking at will appear to be all the same color and texture. Frequently you can't see scratches or defects since they are the same color as the surrounding material. To see these we need some kind or contrast enhancement.

The two commercially used types are dark field and Nomarski. Coupled with bright field and polarization with a universal illuminator this makes a powerful system for inspection and identification.

Reflected light dark field

Reflected light dark field is a very useful technique since it show height differences in the specimen very well. This means that you can see scratches and other defects that are the same color as the specimen quite easily.

In reflected dark field a central stop is placed in the illuminator light path. The remaining light is reflected by an annular mirror through a collar surrounding the objective. The collar usually contains a lens or diffuser to get good even field illumination. There is no half silvered mirror in dark field since an annular mirror is used. The image light goes to the eye pieces with out a mirror.

The light is then directed at the specimen at an extreme oblique angle. No zero order light reaches the objective, only light that has been reflected from an irregular feature of the specimen. A perfectly flat object would appear black with reflected dark field however darn near nothing is perfectly flat.

Since reflected light dark field needs a collar around the objective reflected dark field objectives have much larger diameters and have much larger diameter mounting threads. This means that reflected dark field objectives must be mounted on a special nosepiece. A reflected light dark field objective will not fit on a normal R.M.S. nosepiece. Usually manufacturers will make an adapter to put bight field reflected light objectives on a darkfield nosepiece but not to put darkfield objective on a bright field nosepiece.

All good microscope manufacturers make illuminators that allow you to switch between bright field and dark field techniques. Since dark field objectives are optically just like bright field objectives they do an excellent bright field job. If you need dark field get only dark field objectives and use them for bright field too.

There are several draw backs to dark field modules. Since the objectives are physically much larger in diameter a nosepiece can't hold as many. Some types of objectives, such as immersion objectives, can't be used for dark field so make sure what you want is available.

If you have a reflected light module you will need to check with your representative. In the past illuminators were much more restrictive than now. If you want to do a lot of techniques with your old illuminator and dark field objectives you may not be able to. Compatabilities changed for a lot of manufacturers when they went over to the universal illuminator.
What this means to you

Reflected dark field is very valuable if you are looking for small height differences on a specimen or if the specimen has a lot of height differences.

Reflected light Nomarski

Reflected light Nomarski is used to look at low contrast opaque specimens like semi-conductor wafers and unetched metal samples. Modern reflected light Nomarski modules will include adapters for each objective, Nomarski prisms that fit in the adapters for the objectives to be used for Nomarski, and a polarizer and an analyser. A universal illuminator will have a place for the polarizer and analyser.

The microscope should be set up for polarized light and then the Nomarski prism pushed into place. The one Nomarski prism serves both as condenser prism and objective prism. Each prism is designed to match the NA of the objective it is to be used with.

Now the Nomarski prism is adjusted to produce the darkest image. After you find the darkest image adjust the prism slightly to one side or the other. Adjust the prisms on all the objective the same way. The best way is to make sure that all the objectives are set the same direction from the darkest image. This insures that "hills" are always "hills" and "valleys" are always "valleys".

This technique produces high contrast images that have all the resolution that the NA of the objective can give. Its an absolutely essential technique for many material applications. Since universal illuminators make it easy to set up there is no real reason, besides cost, not to have it.

What you see is a 3-D appearing image. This effect is based on the reflective index of the material. This means that small differences in reflection, like grain boundaries, will appear as large differences in height. Scratches, air bubbles any difference in reflection at all will show up.

This is caused by the action of the Nomarski prism on the polarized light from the illuminator. After the light source produces the light it is passed through a polarizer and only one wave front direction is passed through. The Nomarski prism shears this light into two beams with a shear angle that is less than the resolution limit of the objective.

This means that the specimen is illuminated two wave front of light. After they bounce of the specimen they pass back through the Nomarski prism and are recombined. The two wave fronts produce constructive and destructive interference based on the distance they had to travel. This is why small differences in height are rendered as differences in contrast.

The light then goes through the analyser to the eyepieces. The polarizer-analyser combination is always set to provide complete extinction in bright field. Some system allow wave plates to be inserted in the illuminator side so that color Nomarski can be used. In semi-conductor quality control this can be an advantage.
What this means to you

This technique provides 3-D appearing images with high contrast and high resolution.

Reflected light phase contrast

There really isn't any such thing. At one time some manufacturers made a reflected phase module but reflected Nomarski proved to be much better. I think I may have seen one reflected phase module on a microscope in my life and the user didn't use the phase part.