Q1. What is the difference between incident light measurement and reflected light measurement?
A1. Hand-held light meter has 2 measuring methods: incident light measurement and
reflected light measurement.
  Reflected metering reads the intensity of light reflecting off of the subject, as camera's built-in meter reads, they are easily fooled by variances in tonality, color, contrast, background brightness. Reflected measurements of any single tone area, for instance, will result in a neutral gray rendition of that object because cameras and hand-held light meter can not identify the brightness of subjects. Therefore, the measured exposure value of reflected light measurement depends on the reflectance ratio of subjects. This enables photographer to know the contrast of the subject to see if highlights or shadows are within the latitude of film or dynamic range of DSLR. However, you may need to compensate the measured value depending on the subjects.
  In contrast, the measured value of incident light does not depend on reflectance of subjects or background brightness. because incident metering reads the intensity of light falling on the subject. However, this measurement has to be made from the subject position.
You need to select the measuring method of light meter tailored to the photographing situation.

Q2. What is the difference between Lumisphere and Lumidisc?
A2. Both Lumisphere and Lumidisc can be used for incident light measurement, and you can use them according to the purpose as below. Some light meters from Sekonic offer retracted Lumisphere as Lumidisc.
Extended Lumisphere Extended lumisphere can be used to measure 3 dimensional subjects with Lumisphere aimed in the camera direction at the position of the subject.
Retracted Lumisphere
Retracted lumisphere or lumidisc can be used to measure manuscripts, paintings or other flat copy. (The measurement can be also taken from the position of the subject to the camera direction)It can be also used for measuring illumination levels or brightness difference because it is possible to measure only main light source and avoid receiving the other light sources.
Extended lumisphere   Retracted lumisphere
Q3. Measured value differs in comparison with camera's built-in exposure
A3. Measured exposure value of reflected light measurement depends not only on reflectance of subjects but also on measuring range determined by light receiving angle of light meter. If there are some subjects in extraordinary contrasts, the measured exposure value totally differs by the measuring range.
For example, SEKONIC light meter L-308S has 40 degrees light receiving angle, and L-758D has 1 degree light receiving angle for reflected light measurement.
Although a number of advanced SLRs offer spot-metering capability, the metering angle is directly related to the focal length of the camera lens in use. Every time the lens is changed, the effective spot-meter angle changes.
Under these conditions, it is natural that the measured value of camera's built-in meter can differ from that of hand-held light meter depending on the subjects.
One hand-held light meter can be a master of light meter for any camera・・・35mmSLR, Medium or Large format cameras, and film or digital.
A hand-held 1degree spot meter, on the other hand, allows the most selective measurement of distant subjects, as well as small areas in more complex scenes.
Q4. Are there some substitutes for mercury batteries of old SEKONIC light meters?
A4. Mercury battery is already discontinued because of environmental problem. The voltage of mercury battery (mainly 1.35 V) can differ from the voltage of existing battery (mainly 1.5 V). There are some substitute batteries for mercury batteries to be used with conversion adapter which reduce the voltage.
For example: Mercury battery H-D = MR-9 conversion adapter + silver oxide battery SR-44
Mercury battery HM-4N = Conversion adapter for mercury battery HM-4N + silver oxide battery 4LR-44 or 4SR-44
For details of, please see "Battery used for each light meter" in the page of "SEKONIC HISTORY"

Note: Light meter does not always work fine or measure correctly even with the above conversion adapter and the corresponding battery. Sekonic may not have tools and equipment for repair and calibration for the models which was discontinued over seven years ago. Thank you for your understanding in advance.
Q5. What is EV?
A5. EV (Exposure Value) is the reading that logarithmically expresses the constant quantity of light combined from the shutter speed (Time Value: TV) and aperture (Aperture Value: AV). With 1 EV change, the quantity of light doubles (or halves).
As the following table shows, AV0 starts from F1.0 to larger aperture, and TV0 starts from 1s to faster shutter speed.

For example: There are 9 combinations of shutter speed and aperture for EV 12 in the chart above:
Shutter speed 1/500 s / Aperture 2.8
Shutter speed 1/15 s / Aperture 16 etc
Q6. What is the difference between standard exposure and proper exposure?
A6. There are basically 2 kinds of exposure, standard exposure and proper exposure. Hand-held light meter can offer standard exposure. The photo with standard exposure appears to be looked at by the human eyes. On the other hand, proper exposure is determined by photographers, possibly with compensation from the standard exposure to take photos tailored to their needs of photography or intention. To determine "Exposure" may be eternal theme for photographers.
Q7. Is light meter necessary for digital photographing?
A7. In today's digital age, although the technology behind photography is much more advanced, obtaining professional quality results still require the best techniques and tools.
Digital capture has a dynamic range and latitude roughly comparable to transparency film. What this means is if you are used to properly exposing transparencies, you are off to a good start. But if you are accustomed to the wider exposure latitude (extra margin of exposure error) that color negative film gave you, it is time to change your thinking.
Although it may seem that any miracle can happen in the digital darkroom, correcting a bad exposure has its limits. At first glance it may look like it is possible to make corrections, but upon closer inspection you may notice that image quality has suffered in addition, the time spent on the computer fixing a bad exposure will far exceed the amount of time it takes to get it right in the camera.
While a histogram is the most convenient way to see how the entire scene will reproduce (or not), it really doesn't tell us the whole story. There's information about absolute subject exposure and lighting but it's difficult to interpret. Plus there's no feedback in terms of numbers that match your camera setting (i.e.f/stops) - that's the job of a light meter. In addition, most camera manufacturers only give you an approximate representation of the subject histogram, because they realize the viewing window is too small to provide accurate detail.
Although there are some exceptions to this, Cameras that offer more information are more expensive and/or are designed for the slower paced studio environment where you have lots of time to analyze and reanalyze both lighting and exposure. And don't be fooled into thinking you can determine exposure accurately on the LCD screens on the back of digital cameras. While they generally offer a fair representation of the image, they are most often low resolution and difficult to see, which makes them hard to use to judge subtle subject variation, as well as misleading when viewed under different lighting condition.
The best insurance for proper exposure is a good handheld light meter