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Technical detail according to Baader Source: Teleskop-Express
What is it used for? A Front ERF basicly prevents the solar heat from entering your telescoop. The "cool" D-ERF filters by Baader are unique - and made in Germany! DThe filter glass is, in opposite to other makes, plane-parallel polished with a precision of 1/10 ?. The have an effective infrared reflective coating protecting the tube's internal parts from heat. Tube seeing is reduces and expensive filters near the focus are protrected from thermal stress. This is a prerequisite to use H-a narrow band filters. The D-ERF filters are the successors of Baader's C-ERF filters. The Baader D-ERF main advantages: High quality plane-parallel polished substrate with a surface precision of 1/10 ?. Dielectric interference layer coating system for IR reflection Multi-layer anti-reflective coatings to prevent reflections Stress-relieved substrate Source: Teleskop-Express
Own measurement Date measurement: 16-05-2013 Equipment used: Perkin Elmer Lambda35 Bandwidth : 1nm Not quite what they promised. Enough to exploir why we both get different results. To be continued!
Baader D-ERF D=110mm - Energy Rejection Filter for solar observation  - thermal reduction # 2459243D