For upright microscopes, the size of the sample is limited to an average height of 80 mm and a weight of 3 kg, depending also on the objective that is being used. This limitation does not apply to inverted microscopes. Because the optics are below the stage, the sample is placed above the objectives. This means that users have greater working distance and can work with huge and heavy samples that weigh up to 30 kg. So if you work with big and heavy samples, or with samples that differ considerably in size and weight, inverted microscopes provide you the freedom you need.
Fig. 1: Left: light path of an inverted microscope. Right: an inverted microscope with a large sample.
With an inverted microscope, you simply place your sample on the stage, focus onto the surface once and image it. Finished. The sample stays focused for all magnifications and further samples of the same sort are in focus alike.
With an upright microscope the workflow spans more tasks operators need to perform: You need to lower the stage, move it out, remove the sample holder and take a new one, place the sample on the holder and protect it and use a sample press to level the sample surface, before you change to a lower magnification to position. Operators need to practice all of these steps and, still, all different steps bear the risk of failure every time they are performed.
For untrained operators, the placement of samples can be a daunting task – with an inverted microscope, placing the sample on the stage becomes child’s play and requires less steps to perform as well.
Another point is: All of the steps described for upright microscopy take time that adds up, especially if you have to look at many samples one after the other. Let us assume an experienced user operates an upright microscope. Even if only five seconds per step are needed, a user of the inverted microscope would beat his colleague by far: Below is an example of the user working with an upright microscope versus the user working with an inverted microscope such as the Leica DMi8 for industrial applications. Their task is to analyze a standard metallographic sample.
|Lower the stage||5 sec.||5||–|
|Move the stage out||5 sec.||10||–|
|Remove the sample holder/take a new one||5 sec.||15||–|
|Place the sample holder on the holder and protect it||5 sec.||20||–|
|Use a sample press to level the sample surface||5 sec.||25||–|
|Change to lower magnification for positioning||5 sec.||30||5 sec.||5|
|Place sample back onto the stage, move back, refocus||10 sec.||40||5||10|
Tab. 1: Workflow for upright versus inverted microscopes.
According to this assumption, the inverted microscopes enables you to change up to four times faster between samples compared to analysis on an upright microscope, so you can reach a higher throughput with an inverted microscope.
Crashing your objective into your sample happens to the best of us. It is a well-known risk with upright microscopes. When this happens, not only do you have to invest in a new objective but also will you have to discard the sample. In most cases no spare objective will be instantly available but needs to be ordered instead. This results in reduced sample throughput and in the worst case can interrupt work completely if there is no second microscope available.
The design of an inverted microscope helps reduce the risk of crashing the objective into your sample considerably. Firstly, the objectives are positioned below the stage and thereby more protected. Secondly, instruments such as the Leica DMi8 for industrial applications have an upper focus stop functionality which provides additional security by defining the upper limit of the nosepiece. So while protecting your investment, you can focus on your task instead of spending time worrying about possible damage to the tool and sample.
Sample preparation is limited, because the sample can be taken as is and put on the stage. Additional, only one side of the sample needs to be processed. You save time as you do not need to embed the sample, nor do you need to cut out parts of bigger samples. While handling the sample on the microscope, there is no need to level the sample using a sample press. Instead of seven sample preparation steps, you only need to do two. And this saves you time and money.
Fig. 2: Left: Sample with polished surface for an inverted microscope. Right: For an upright microscope, the sample has to be mounted on a sample holder.
If you observe a sample with an upright microscopes, your brain has a nut to crack: When you move the stage to the left, the image of the sample that you see through the eyepieces moves to the right due to the construction of the instrument. And vice versa, of course.
If you observe a sample with an inverted microscope such as the Leica DMi8 for industrial applications, the image of the sample moves in the same direction as the stage. It behaves the way we have learned things move in the world. Especially for untrained users this is a real alleviation of their work, because they do not have to think about the direction in which they want to move the sample, but just do it as if they had no optics to look through. And whatever you don’t have to think about while doing it saves time and therefore speeds up your process.